How Belief Works is an ongoing series of articles on the psychology of belief that's best read in sequence.

The correct answer to the question of how beliefs form is even simpler than the two answers I considered in the previous article, but is, unlike those two answers, initially highly counterintuitive. It was first presented more than three centuries ago, but has only ever been supported by a small minority of scholars. And yet, just as logic alone can disprove the answer that the formation of belief X is us assessing that claim X is true, logic alone can prove the correct answer.

And the truth about how beliefs form has profound implications for our understanding of human psychology, and therefore also for our lives.

A radical theory of belief formation

This theory of belief formation was originally proposed by the prominent 17th-century philosopher Baruch Spinoza, in his posthumous 1677 book Ethics.

Consider that a claim enters our mind via one of four cognitive processes:

• comprehension – when the claim is communicated to us via some medium that we perceive, which is normally voice or text

• reason – when the claim is a conclusion

• imagination – when we generate the claim without using reason

• recollection – when we retrieve the claim from our memory.

Spinoza’s theory of belief formation is that the mere entrance of a claim into our mind causes us to believe it.

This theory can seem obviously wrong – which can itself seem to disprove it. It can seem obvious from experience that merely comprehending, recalling or imagining-up a claim doesn't cause us to believe it. Regarding reason, a conclusion is by definition a belief. But the theory implies that our belief of this claim upon it entering our mind is due merely to that entrance, rather than being due to the claim being the product of our reason. And that can seem obviously contrary to both the concept and experience of reasoning.

But closer analysis reveals that Spinoza's theory actually isn’t contrary to our experience of claims entering our mind via any of these processes. And although the theory is indeed contrary to the current concept of reasoning, closer analysis reveals that this concept can be revised to agree with the theory.

Also, there’s increasing experimental support for the theory. For example, a 2022 paper by neuroscientists at the universities of Harvard, Princeton and Dartmouth in the US presents brain scan evidence which seems to support the theory.

And the first academic conference on Spinozan belief formation took place in 2018, in the US.

It might be thought that the failure of Spinoza's theory to gain wide acceptance by scholars after more than three centuries suggests, in itself, that it's wrong. However, the correct theory that the Earth goes around the Sun, rather than vice versa, was also initially highly counterintuitive – given the Sun's daily apparent motion across the sky, and our inability to feel the Earth's actual motion – and only became widely accepted by scholars more than nineteen centuries after it was first proposed, by the ancient Greek astronomer and mathematician Aristarchus of Samos.

To see why Spinoza was also right, consider first the two ways that a claim can be involved in our thinking.

The two ways that a claim can be involved in our thinking

If claim X is involved in our thinking then we're either thinking X or thinking about X. For example, if the claim David is vegetarian is involved in our thinking then we're either thinking 'David is vegetarian' or thinking about the claim David is vegetarian.

Thinking claim X is the mental state of believing X. For example, thinking 'David is vegetarian' is the mental state of believing that David is vegetarian. Indeed, we often use the word think to refer to the mental state of belief. For example, we may refer to Amy’s mental state of believing that David is vegetarian by saying 'Amy thinks that David is vegetarian'.

Consider the mental state of concluding X. By definition, concluding X involves thinking X. For example, by definition, concluding 'David is vegetarian' involves thinking 'David is vegetarian'. And, also by definition, to conclude X is to believe X. 

We can of course continue to hold belief X after thinking X, when our belief exists not as a mental state with an associated brain state but solely as a brain state, in a memory area of the brain. So believing X doesn't necessarily involve currently thinking X. That is, the psychological state of belief doesn't always involve the mental state of belief, although it always begins with that mental state, however briefly.

According to some psychologists and philosophers, we actually don’t think in the language, or languages, that we communicate in. Instead, the medium of thought is a universal mental language called the language of thought or mentalese. But the logic of the argument presented in this article – and of that presented in the next – equally applies to this understanding of thought.

Regarding thinking about claim X, we obviously can do this while not believing X. For example, we obviously can think about the claim David is vegetarian while believing that David isn't vegetarian, or while not holding a belief about whether David is vegetarian. This fact can seem to disprove Spinoza's theory, but, as I'll explain later, it doesn't.

We can actually think about a claim without it even existing in our mind. For example, if we know that David told Amy what his dietary restrictions are, but don't know what he said, then this claim doesn't exist in our mind, but we can think about it in the sense of thinking about its existence in David and Amy's minds. But I'm concerned in this argument with thinking about claim X in the sense that involves X existing in our mind, as thinking X does. And it’s this sense that I mean when I subsequently refer to thinking about X.

It might be thought that thinking 'I believe claim X' is another form of the mental state of believing X, and so the mental state of believing X can involve thinking about X. But thinking 'I believe claim X' is actually the mental state of believing that we believe X. Likewise, it might be thought that saying 'I believe claim X' is an expression of our belief of X, and so the mental state of believing X can involve thinking about X. But saying 'I believe claim X' is actually our expression of our belief that we believe X. Our expression of simply our belief of X instead involves saying simply X.

In the previous article I explained that claim X and the claim X is true are different claims, and so belief X and the belief X is true are by definition different beliefs. The difference between these beliefs aligns with the difference between thinking X and thinking about X. That is, whereas the mental state of believing X involves thinking X, the mental state of believing X is true involves thinking 'X is true', and thinking that claim X is a true claim involves thinking about X.

Now consider how the content of claim X exists in our mind as we’re thinking X and thinking about X.

The two ways that the content of a claim can exist in our mind

A claim can, of course, be about anything. But all claims are ultimately about the same thing: reality. The definition of a true claim is a claim that matches reality, and the definition of a false claim is a claim that doesn't match reality.

Reality includes the content of our own mind and the minds of others. So the content of our imagination, which is often contrasted with reality, is actually part of reality in the sense that it's part of mental reality. For example, the content of a fictional story that we're reading is part of reality in the sense that it exists in our imagination and is therefore part of mental reality. Concepts, being abstract, are also often considered distinct from reality. But concepts are ideas and so exist in our mind, and so are also part of reality in the sense of being part of mental reality.

A claim is analogous to a picture, in the sense that the content of both is a representation of an aspect of reality, however accurate. Indeed, we sometimes say that a particular description 'paints a picture'. Even if a picture is of a scene that existed only in the imagination of the picture’s creator, the content of that picture is a representation of the content of that imagination, which is part of mental reality.

There are of course different ways to create a picture, such as painting, drawing, using art software, or photography. Consider a painting of an outdoor scene.

The painting is ultimately just a collection of patches of paint on a surface. The picture we see, of an outdoor scene, as we look at this collection of patches of paint is our brain's interpretation of this collection of patches of paint. That is, the picture isn't the painting but what forms in our vision as we look at this collection of patches of paint. However, because we see this collection of patches of paint as a picture, the painting and the picture seem to be the same thing.

So what we refer to as 'a painting of an outdoor scene' isn't itself a picture of an outdoor scene, and is instead a painting that creates in our vision a picture of an outdoor scene. What we refer to as 'a photorealistic painting of an outdoor scene' therefore actually isn't itself photorealistic, and is instead a collection of patches of paint that creates in our vision a photorealistic picture of an outdoor scene

The relationship between this picture and the painting is analogous to the relationship between a claim and the sentence that expresses it. Such a sentence is a declarative sentence – a sentence that declares something, as opposed to one that expresses either a question, advice, an instruction, a command, a request or an exclamation.

A declarative sentence is ultimately just a string of words, but together they express a claim. That is, the claim isn't the sentence – the string of words – but what is asserted by the sentence. In philosophy and psychology the term proposition is normally used instead of claim. And Oxford University Press's A Dictionary of Psychology defines this sense of the term proposition as:

The meaning conveyed by a declarative sentence ... The proposition is not the sentence itself but its meaning. ...

For example, when we refer to 'the claim David is vegetarian' we're referring not to the sentence David is vegetarian, but to what's asserted by this sentence. We refer to 'the claim David is vegetarian’, even though David is vegetarian is a sentence, because, by definition, we can only ever express this claim by using a sentence that expresses it. But doing so can lead to the claim and the sentence being conflated.

The difference between the sentence David is vegetarian and the claim David is vegetarian is apparent from the fact that, by definition, this claim is also made by other sentences which have the same meaning, such as David follows a vegetarian diet, David adheres to vegetarianism, David abstains from eating meat, David est végétarien and David ist vegetarisch. Logically, if sentence X and claim X were the same thing then that claim couldn't also be the same thing as a different sentence.

Analogously, as we look at the painting of an outdoor scene, the picture we see is the same as the picture we see as we look at a print of this painting, even though the painting is made-up of paint brushstrokes whereas the print is made-up of microscopic dots of ink, and even if the dimensions of the print are different from those of the painting. So the painting and the picture are indeed not the same thing. That is, logically, if the painting and the picture were the same thing then the picture couldn't also be the same thing as the print, which is different from the painting.

Also, there can be multiple legitimate interpretations of what claim is being made by a sentence. For example, the sentence Emma likes racing cars can be interpreted as making either the claim that Emma likes cars designed for racing, or the claim that Emma likes racing in cars. Logically, if sentence X and claim X were the same thing then that sentence couldn't also be the same thing as a different claim.

Analogously, when we look at this drawing – the 'rabbit-duck illusion' – we see a picture of the head of either a rabbit – facing right – or a duck – facing left:

So each picture is indeed not the drawing itself. That is, logically, if the drawing and either picture were the same thing then the drawing couldn't also be the same thing as the other, different, picture.

Given that the claim David is vegetarian is what is asserted by the sentence David is vegetarian, it’s the content of this sentence. And the content of a declarative sentence is an idea, a concept. So the claim David is vegetarian is the concept that David is vegetarian.

It might be thought that the claim that David is vegetarian and the concept that David is vegetarian are different things. That is, it might be thought that whereas the former involves someone claiming that David is vegetarian, the latter doesn't, because it's merely the idea that David is vegetarian.

But consider that claim X can be generated and communicated without anyone actually claiming X. For example, if we make-up the claim David is vegetarian, and then present it to others as a claim that we've made-up, then we've generated and communicated this claim without actually claiming that David is vegetarian.

So although a claim is by definition asserted by a declarative sentence, it hasn't necessarily been asserted by a person. That is, a claim, an assertion, is in itself merely a concept, an idea. Indeed, we refer to the concept that David is vegetarian using – as I just did – the sentence David is vegetarian, which expresses the claim that David is vegetarian.

So the claim that David is vegetarian actually doesn't in itself involve someone claiming that David is vegetarian, and is instead merely the concept, the idea, that David is vegetarian.

Of course, not all concepts are claims. For example, the concept of vegetarianism is the concept of abstinence from meat, and the words vegetarianism and abstinence from meat don't express claims – that is, they don't form declarative sentences. But all claims are concepts.

The fact that a claim is a concept means that, whereas a declarative sentence and its content – a claim – aren’t the same thing, a claim and its content are. For example, the content of the claim David is vegetarian is the concept that David is vegetarian, which is the claim David is vegetarian. So this claim and its content are simply the same thing, rather than the latter being what is asserted by the former.

Indeed, to say that the claim David is vegetarian matches reality is in itself to say that the content of this claim matches reality. And to say that the claim David is vegetarian is what is asserted by the sentence David is vegetarian is in itself to say that the content of this claim is what is asserted by this sentence.

Analogously, whereas the painting of an outdoor scene and its content – the picture of that scene – aren’t the same thing, the picture and its content are. An aspect of the picture – as opposed to an aspect of the painting, such as the brushwork – is an aspect of the content of the picture. And the picture as a whole is the whole content of the picture.

Indeed, to say that the picture is an accurate representation of the outdoor scene is in itself to say that the content of the picture is an accurate representation of that scene. And to say that the picture is what we see as we look at the painting is in itself to stay that the content of the picture is what is what we see as we look at the painting.

In the case of the rabbit-duck illusion, to stay that the picture changes depending on how our brain interprets the drawing is in itself to stay that the content of the picture changes depending on how our brain interprets the drawing.

An advantage of referring to, for example, ‘the content of the claim David is vegetarian’ rather than just ‘the claim David is vegetarian’ is that the latter wording, unlike the former, could be misunderstood as referring to the sentence David is vegetarian rather than the content of this sentence. Analogously, in the case of the picture we see as we look at the painting of an outdoor scene, an advantage of referring to ‘the content of the picture’ rather than just ‘the picture’ is that the latter wording, unlike the former, could be misunderstood as referring to the painting rather than the picture.

Indeed, consider the wording 'the claim David is vegetarian is the concept that David is vegetarian'.

Again, the content of this wording can seem wrong because it can seem that the claim that David is vegetarian involves someone claiming that David is vegetarian, whereas the concept that David is vegetarian doesn't, because it's merely the idea that David is vegetarian. But consider the alternative wording 'the content of the claim David is vegetarian is the concept that David is vegetarian'. The content of this wording doesn't seem wrong for the above reason even for someone who wrongly believes that claim X involves in itself someone claiming X.

However, the content of the original wording can also seem wrong because that wording can be misunderstood as saying that the sentence David is vegetarian is the concept that David is vegetarian, when the latter is obviously the content of the former. But consider again the alternative wording 'the content of the claim David is vegetarian is the concept that David is vegetarian'. The content of this wording doesn't seem wrong for the above reason even for someone who wrongly conflates claim X and sentence X.

Also note that the concept that David is vegetarian can also be referred to as the concept of David being vegetarian. So the claim David is vegetarian can be also be referred to as the concept of David being vegetarian. Alternatively stated, the content of the claim David is vegetarian can also be referred to as the concept of David being vegetarian.

Now consider the following thought experiment.

Imagine a room with a fake window. The fake window, on an external wall, consists of a real window frame surrounding a photorealistic painting of a daytime outdoor scene, without any space between the edge of the painting and inside edge of the frame. It's located opposite the room's entrance, so when someone enters the room they’re looking at it face-on. And the scene they see within the frame, in that moment, is what they'd see from the room's entrance, in the daytime, through a real window in the same location. Also, that scene is one in which an absence of movement isn't unusual.

Now imagine entering the room for the first time, during the daytime, with the room's lighting on, and without knowledge of the fake window. If upon us looking at the fake window it creates the illusion of a real window, at least initially, then our visual experience will consist of us apparently directly seeing a real outdoor scene through a real window. And if the fake window doesn't create this illusion, even initially, then our visual experience will consist of us seeing merely a picture of an outdoor scene, in a fake window.

So upon us looking at the fake window the content of the picture exists in our vision either as the content of a picture of an outdoor scene or as a real outdoor scene.

These two ways that the content of the picture can exist in our vision are the only two logically possible ways that it can exist in our vision. If the content of the picture doesn't exist in our vision as the content of a picture of an outdoor scene then the only other logical possibility is that it exists in our vision as a real outdoor scene – and vice versa. That is, if our visual experience doesn’t consist of us seeing a picture of an outdoor scene then the only other logical possibility is that it consists of us apparently directly seeing a real outdoor scene – and vice versa.

And these two, and only two, logically possible ways that the content of the picture can exist in our vision are analogous to the two, and only two, logically possible ways that the content of a claim can exist in our mind.

The picture's content existing in our vision as the content of a picture is analogous to a claim's content existing in our mind as the content of a claim. And it might be thought that when a claim exists in our mind the only conceivable way for its content to exist in our mind is as the content of a claim. But without consideration of the above thought experiment it might similarly be thought, regarding the picture that exists in our vision as we look at a pictorial painting, that the only conceivable way for its content to exist in our vision is as the content of a picture.

Consider assessing that claim X is a true claim.

As I explained in the previous article, this assessment is based on our belief of X. By definition, a claim is a true claim if, and only if, it matches reality. So the only way to assess whether a claim is a true claim is to assess whether it matches reality. And the only way to assess whether a claim matches reality is to compare it with reality. But, logically, when we compare a claim with reality we can only ever compare it with what we believe about the relevant aspect of reality at the moment of the comparison, even when the comparison uses the current content of our senses.

So our assessment that claim X is a true claim is based on us having a conception of both the content of claim X and the relevant aspect of reality, and then concluding that they match. That is, that assessment is based on the content of claim X existing in our mind as both the content of a claim and an aspect of reality. For example, our assessment that the claim David is vegetarian is a true claim is based on the content of this claim existing in our mind as both the content of a claim and an aspect of reality.

In the case of assessing that claim X is a false claim, this assessment is based on us having a conception of both the content of claim X and the relevant aspect of reality, and then concluding that they don't match. That is, that assessment is based on the content of claim X existing in our mind merely as the content of a claim.

So just as, in the fake window thought experiment, the content of the picture can exist in our vision as the content of a picture of an outdoor scene or as a real outdoor scene, the content of a claim can exist in our mind as the content of a claim or as an aspect of reality.

Again, a picture and its content are the same thing, and a claim and its content are the same thing. So it can alternatively be said that, just as, in the fake window thought experiment, the picture can exist in our vision as a picture of an outdoor scene or as a real outdoor scene, a claim can exist in our mind as a claim or as an aspect of reality.

In the case of the fake window thought experiment, the content of the picture exists in our vision as either the content of a picture of an outdoor scene or a real outdoor scene, whereas, in the case of assessing that claim X is a true claim, the content of X exists in our mind as both the content of a claim and an aspect of reality. But this difference doesn't undermine the point that the two ways that the content of the picture can exist in our vision are analogous to the two ways that the content of claim X can exist in our mind.

And the existence of the content of claim X in our mind as an aspect of reality is the mental state of believing X. Indeed, the mental state of believing X involves thinking X, which, as a matter of logic, can't in itself involve the content of X existing in our mind as the content of a claim.

The content of X existing in our mind as the content of a claim involves, by definition, thinking about claim X. For example, the content of the claim David is vegetarian existing in our mind as the content of a claim involves, by definition, thinking about this claim. Indeed, given that a claim and its content are the same thing, the content of X existing in our mind as the content of a claim is that content existing in our mind as a claim, which by definition involves thinking about X.

So although thinking X involves, by definition, the content of X existing in our mind, it can't in itself involve that content existing in our mind as the content of a claim, given the difference between thinking X and thinking about X. For example, although thinking ‘David is vegetarian’ involves, by definition, the content of this claim existing in our mind, it can’t in itself involve that content existing in our mind as the content of a claim.

Given that a claim and its content are the same thing, it can alternatively be said that although thinking X involves, by definition, X existing in our mind, it can't in itself involve this claim existing in our mind as a claim, given that that would by definition involve thinking about X. For example, although thinking ‘David is vegetarian’ involves, by definition, this claim existing in our mind, it can’t in itself involve this claim existing in our mind as a claim.

Analogously, regarding the fake window thought experiment, given that a picture and its content are the same thing, instead of saying that the content of the picture can exist in our vision without existing in our vision as the content of a picture, it can be said that the picture can exist in our vision without existing in our vision as a picture. But without consideration of this thought experiment it might be thought, regarding the picture that exists in our vision as we look at a pictorial painting, that the only conceivable way for the picture to exist in our vision is as a picture.

Also, again, a claim is a concept. So it can also alternatively be said that although thinking X involves, by definition, concept X existing in our mind, it can't in itself involve this concept existing in our mind as a concept. For example, although thinking ‘David is vegetarian’ involves, by definition, the concept of David being vegetarian existing in our mind, it can’t in itself involve this concept existing in our mind as a concept.

Given the speediness of the brain, which functions on the timescale of milliseconds, we may think about claim X so soon before or after thinking X that our thinking X can seem to in itself involve thinking about X. Thus, thinking X can seem to in itself involve the content of X existing in our mind as the content of a claim. That is, thinking X can seem to in itself involve this claim existing in our mind as a claim – it can seem to in itself involve this concept existing in our mind as a concept.

It might be thought that if as we’re thinking ‘David is vegetarian’ the content of this claim can’t exist in our mind as the content of this claim, because that would involve thinking about this claim, then perhaps it exists in our mind as the content of a claim without existing in our mind as the content of a specific claim, and so doesn't involve thinking about the claim David is vegetarian.

However, given that a claim and its content are the same thing, the existence of the content of the claim David is vegetarian in our mind as the content of a claim is the existence of the claim David is vegetarian in our mind as a claim – which by definition involves thinking about this claim. That is, the content of the claim David is vegetarian can’t exist in our mind as the content of a claim without existing in our mind as the content of the specific claim David is vegetarian.

Analogously, in the fake window thought experiment, the content of the picture obviously can’t exist in our vision as the content of a picture without existing in our vision as the content of that specific picture.

If thinking claim X can't in itself involve the content of X existing in our mind as the content of a claim – if it can't in itself involve this claim existing in our mind as a claim, if it can't in itself involve this concept existing in our mind as a concept – then the only other logical possibility is that it involves the content of X – this claim, this concept – existing in our mind as an aspect of reality.

That is, given that all claims are about an aspect of reality, if thinking X doesn’t in itself involve the content of X existing in our mind as the content of a claim about an aspect of reality, then the only other logical possibility is that it involves that content existing in our mind simply as that aspect of reality. For example, if thinking ‘David is vegetarian’ doesn’t in itself involve the content of this claim existing in our mind as the content of a claim about an aspect of reality, then the only other logical possibility is that it involves that content existing in our mind simply as that aspect of reality.

And the conclusion that thinking X must involve the content of X existing in our mind as an aspect of reality obviously accords with the fact that thinking X is the mental state of believing X.

Regarding the conclusion that thinking claim X involves this concept existing in our mind not as a concept, but as an aspect of reality, it might be thought that, by definition, the content of an abstract claim, such as The death penalty is wrong, can only exist in our mind as a concept. That is, it might be thought that the content of the claim The death penalty is wrong can only exist in our mind as the concept that the death penalty is wrong. So thinking/believing 'The death penalty is wrong' must involve the content of this claim existing in our mind as a concept that's an aspect of reality.

However, given that the concept that the death penalty is wrong is the claim The death penalty is wrong, this concept existing in our mind as a concept is this concept existing in our mind as a claim, which involves thinking about this claim. So thinking even this abstract claim can't in itself involve its content existing in our mind as a concept. That is, thinking/believing 'The death penalty is wrong' must in itself involve the content of this moral claim existing in our mind not as a moral concept that's an aspect of reality, but simply as an aspect of morality reality.

The content of claim X, abstract or not, existing in our mind as an aspect of reality, as we’re thinking/believing X, isn't the mental state of thinking/believing that the content of X is an aspect of reality. The latter involves thinking not simply X, but 'The content of claim X is an aspect of reality'. And it therefore involves thinking about X, with the content of X existing in our mind as the content of a claim. Indeed, it's the mental state of thinking/believing that X is a true claim.

Also, the content of X existing in our mind as an aspect of reality, as we’re thinking/believing X, is different from the existence of that content in our mind as the content of a claim that we believe. The latter involves thinking/believing not simply X, but ‘I believe X’. That is, it's the mental state of believing that we believe X. And it therefore involves thinking about X, with the content of X existing in our mind as the content of a claim.

Perhaps the best way to convey the idea of the content of claim X existing in our mind as an aspect of reality is to refer, as I did, to the fact that assessing that X is a true claim is based on us having a conception of both the content of X and the relevant aspect of reality, and then concluding that they match. And the fake window thought experiment, with the content of the picture possibly existing in our vision not as the content of a picture of an outdoor scene but as a real outdoor scene, helps convey this idea by analogy.

The difference between thinking X and thinking about X has the logical, but counterintuitive, implication that a claim can exist in our mind without us thinking about it. That is, although thinking X involves, by definition, X existing in our mind, it doesn’t in itself involve thinking about X. If we're thinking about X while apparently thinking X then we're actually not at that moment thinking X. Although, again, given the brain's speediness, we may think about X so soon before or after thinking X that our thinking X can seem to in itself involve thinking about X.

But, despite this logic, it can still seem contradictory to say that, for example, thinking ‘David is vegetarian’ doesn't in itself involve thinking about the claim that David is vegetarian. Indeed, thinking any claim necessarily involves thinking about something, and if thinking ‘David is vegetarian’ doesn't in itself involve thinking about the claim that David is vegetarian then what else could it involve thinking about?

This apparent paradox is resolved when it's understood that although thinking ‘David is vegetarian’ involves, by definition, this claim existing in our mind, the claim exists in our mind not as a claim, but as an aspect of reality. That is, although thinking ‘David is vegetarian’ involves thinking about David being vegetarian, David being vegetarian exists in our mind not as a mere claim, but as an aspect of reality.

So the mental state of believing claim X doesn't in itself actually involve thinking about this claim. For example, the mental state of believing the claim David is vegetarian doesn't in itself actually involve thinking about this claim. Again, thinking 'I believe the claim David is vegetarian', which does involve thinking about the claim David is vegetarian, is the mental state not of believing the claim David is vegetarian, but of believing that we believe the claim David is vegetarian.

Note that although the content of both a picture and a claim is a representation of an aspect of reality, however accurate, pictures and claims are themselves aspects of reality. Again, the picture we see as we look at a painting of an outdoor scene is what forms in our vision as we look at this collection of patches of paint on a surface. So that picture is an aspect of mental reality. And a claim, being a concept, is an idea, and so is also an aspect of mental reality.

Indeed, a picture can part of the scene in another picture, and a claim can be referred to by another claim. Also, a picture can be referred to by a claim, and the sentence expressing a claim can appear in a picture.

So, in the fake window thought experiment, although the content of the picture can exist in our vision either as the content of a picture of an outdoor scene or as a real outdoor scene, both involve that content existing in our vision as an aspect of reality. By definition, anything that exists in our vision does so at least as an aspect of mental reality. In the case of seeing a picture, again, the fact that we see this collection of patches of paint as a picture means that the painting and the picture can seem to be the same thing. So the picture that forms in our vision seems to be a part of the scene in front of us.

Likewise, although I've been saying that the content of claim X can exist in our mind as the content of a claim or as an aspect of reality, the former is actually also the content of X existing in our mind as an aspect of reality, albeit in a different way. By definition, anything that exists in our mind does so at least as an aspect of mental reality. I've been referring to the content of X existing in our mind as an aspect of reality to solely mean that content existing in our mind not as merely the content of this claim about an aspect of reality, but as that aspect of reality – and for conciseness I'll continue to do so.

The analysis in this section provides a formal definition of belief: a psychological state in which the content of a claim exists in the mind, or at least memory, as an aspect of reality. Or, given the previous point, more strictly correctly: a psychological state in which the content of a claim exists in the mind, or at least memory, not as the content of this claim about an aspect of reality, but as that aspect of reality.

Thinking X is always believing X

It might be thought that thinking claim X actually isn’t always the mental state of believing X. That is, it can seem obvious that if we don't believe X – whether we believe a contrary claim or don't hold a belief on the subject of X – but we deliberately think X, like a silent mantra, doing so won't cause us to believe X. For example, it can seem obvious that if we believe that David isn’t vegetarian, but we deliberately think 'David is vegetarian', like a silent mantra, doing so won't cause us to believe that David is vegetarian.

I've indeed so far only been concerned with organically thinking X – that is, when our thinking X is either a recollection of an apparent fact or a conclusion, each of which are by definition the mental state of believing X. But the conclusion in the previous section that thinking X necessarily involves the content of X existing in our mind as an aspect of reality, which is the mental state of believing X, equally applies to deliberately thinking X. And this isn’t, upon analysis, contrary to our experience of deliberately thinking X.

Thinking X is different from thinking about X whether the former is organic or deliberate. For example, although deliberately thinking 'David is vegetarian' involves, by definition, thinking about this claim immediately beforehand, and we may also think about it immediately afterwards, doing so doesn’t in itself involve thinking about it.

If we're thinking about claim X while apparently deliberately thinking X then we're actually not at that moment thinking X. Although, given the brain's speediness, we can think about X so soon before, and possibly after, deliberately thinking X that our deliberately thinking X can seem to in itself involve thinking about X. Also, we may think about X during our attempt to think X, which can add to the sense that deliberately thinking X involves, in itself, thinking about X, even though that thinking about X actually interrupts our thinking X.

And, again, the content of X existing in our mind as the content of a claim involves, by definition, thinking about X. So although deliberately thinking X involves, by definition, the content of X existing in our mind, it can't in itself involve that content existing in our mind as the content of a claim.

And the only two logically possible ways that the content of a claim can exist in our mind are as the content of a claim or as an aspect of reality. So deliberately thinking X must involve the content of X existing in our mind as an aspect of reality.

And the content of X existing in our mind as an aspect of reality is the mental state of believing X. So even deliberately thinking X is the mental state of believing X.

And this belief continues beyond us deliberately thinking X, due to the content of X being stored in our memory as an aspect of reality. Again, believing X doesn't necessarily involve currently thinking X.

Even if we begin thinking about X within a fraction of a second of us deliberately thinking X, and the content of X therefore then exists in our mind as the content of a claim rather than as an aspect of reality, this isn't the end of our belief of X. That is, although we can think about X while not believing X, we can of course also think about X while believing X.

So why can it seem obvious that we won't start believing X simply by deliberately thinking X? Consider again the example of believing that David isn’t vegetarian and then deliberately thinking 'David is vegetarian'.

Immediately after deliberately thinking 'David is vegetarian', and thereby believing this claim, we’ll be aware that we believed the opposite immediately before doing so, and that we were thinking 'David is vegetarian' just then only because we decided to say to ourself this claim that we didn't believe. And so we’ll then conclude 'David isn't vegetarian'.

And given the simplicity of this reasoning, and the brain's speediness, it'll occur within a fraction of a second of us deliberately thinking 'David is vegetarian'. So our belief that David is vegetarian will only last a fraction of a second, and we'll therefore normally have no awareness of it.

And the same applies whenever we deliberately think a claim that's contrary to what we believed immediately before doing so.

We may also deliberately think, and thereby believe, claim X when we don't already hold a belief on the subject of X. Immediately after doing so we’ll be aware that we didn’t hold a belief on the subject of X immediately before doing so, and that we were thinking X just then only because we decided to say to ourself this claim that we didn't believe. And so we’ll then conclude that we're ignorant about the truth of X, with this belief therefore replacing our belief of X.

And given the simplicity of this reasoning, and the brain's speediness, it'll occur within a fraction of a second of us deliberately thinking X. So our belief of X will only last a fraction of a second, and we'll therefore normally have no awareness of it.

We may even deliberately think, and thereby believe, claim X when we already believe X – and our belief of X upon and just after doing so will seem to be due to our prior belief of X.

The critical point from the analysis so far is that however we come to be thinking claim X, doing so is always the mental state of believing X. Now consider the entrance of a claim into our mind.

The entrance of a claim into our mind

Again, a claim enters our mind via one of four cognitive processes:

• comprehension – when the claim is communicated to us via some medium that we perceive, which is normally voice or text

• reason – when the claim is a conclusion

• imagination – when we generate the claim without using reason

• recollection – when we retrieve the claim from our memory.

And if claim X exists in our mind we must be either thinking X or thinking about X, given that there’s no other possibility. So the entrance of X into our mind must consist of us either thinking X or thinking about X. But consider the latter possibility.

Thinking about X involves either contemplating X or thinking a claim concerning X. For example, thinking about the claim David is vegetarian involves either contemplating this claim or thinking a claim such as The claim 'David is vegetarian' is false or Amy said that David is vegetarian.

However, I'm concerned here with X entering our mind by itself, upon being comprehended, concluded, imagined-up or recalled. So the question of whether that entrance consists of us thinking X or thinking about X only concerns thinking about X in the sense of contemplating X – which is the sense that I mean when I subsequently refer to thinking about X, except when I'm explicitly referring to thinking a claim concerning X.

Regarding the process of comprehending claim X, it might be thought that this whole process involves, by definition, thinking about this claim that we're trying to comprehend. But this process involves our sense organs collecting information about whatever medium is communicating X, and then the sensory and language areas of our brain analysing that information in order to determine X. So we're unable to think about X until it has been produced by this process. During that process we can only think about X in the sense of thinking about the existence of this claim that we’re trying to comprehend.

Of course, when we’re part-way through comprehending X we can think about the part of X that we’ve comprehended so far. But thinking about X as a whole – in the sense that doesn’t just involve thinking about the existence of X, but involves X existing in our mind – obviously requires us to have comprehended the whole of X.

Regardless of the process by which claim X enters our mind, thinking about X is a further cognitive process. Any kind of process involves the processing of the input or inputs to the process, and, possibly, the production of an output by that processing:

The inputting of something into a process is the beginning of the processing of that thing, and the ouputting of something from a process is the completion of the production of that thing.

For example, the process of book manufacturing involves the processing of paper, ink and other materials – the inputs – to produce a book – the output. In the case of the cognitive process of thinking about X, X is obviously the main input to this process, with the other inputs being related things stored in our memory. And any output of this process is thought Y about X, whether Y is generated by our reason or imagination, or is recalled.

An input to a process can of course itself be the output of another process:

In the case of book manufacturing, the paper, ink and other materials used to produce a book are themselves the outputs of other processes. In the case of thinking about X, X is the output of the cognitive process of either comprehension, recollection, reason or imagination:

And the outputting of X from that first process is by definition the entrance of X into our mind. That is, the completion of the cognitive process of comprehending, recalling, concluding or imagining-up a claim is by definition the entrance of that claim into our mind.

It's possible that upon X being created, recreated or retrieved by the first process, X must then be transmitted to some central processing area of the brain in order to enter the mind. And that transmission of X to this area would itself be a process. But this process would be the final sub-process of the first process, given that the completion of the first process is by definition the entrance of X into our mind. That is, the outputting of X from the first process would be the production of X in this central processing area.

So in order for the entrance of X into our mind to consist of us thinking about X – that is, in order for the entrance of X into our mind to be the entrance of X into the process of thinking about X – the outputting of X from the first process must also be, in itself, the inputting of X into the process of thinking about X. That is, the outputting of X from the first process, and the inputting of X into the second, must be different perspectives of the same event, rather than different events.

However, the outputting of something from one process actually can't ever be the same event as the inputting of that thing into a second process.

It might be thought that the outputting of claim X from the first process also being, in itself, the inputting of X into the process of thinking about X is analogous to the exiting of someone from a room, via a doorway to another room, also being, in itself, their entering the other room. That is, their exiting the first room, and their entering the second, are different perspectives of the same event – them crossing the threshold – rather than different events. However, this is actually a false analogy.

Someone walking through a doorway constitutes one process – the process of walking through a doorway – and so not two different processes. Indeed, the moment when the person's walking involves them crossing the threshold isn't them being either outputted from a process or inputted into a second, different, process. That is, this event isn't either the completion of the production of this person by a process or the beginning of the processing of this person by second, different, process.

Someone exiting any area of the physical world – in the sense that involves them moving through physical space out of that area, and doesn't involve them exiting the physical world itself – necessarily involves, in itself, them entering another area of the physical world – in the sense that involves them moving through physical space into that area, and doesn't involve them entering the physical world itself – and vice versa.

Even when someone walks from one room to another via a corridor, their exiting the first room is also them entering the corridor, and their entering the second room is also them exiting the corridor. Even when someone walks out of a building, their exiting the building is also them entering the area outside the building.

Someone exiting an area of the physical world, and them entering another area of the physical world, are different perspectives of the same event: them crossing the boundary between two adjacent areas of the physical world. So neither can occur without it involving, in itself, the other. It isn't even possible to imagine either occurring without it involving, in itself, the other.

But it's possible to imagine the outputting of something from one process, and the inputting of that thing into a second process, being separate events. That is, it's possible to imagine the processing of this thing by the second process not beginning for a period of time after the completion of the production of it by the first process.

And if it's possible to imagine the outputting and inputting of the same thing occurring without either involving, in themselves, the other, then, by definition, they're different kinds of events.

Indeed, whereas someone exiting an area of the physical world, and them entering another area of the physical world, are merely different perspectives of the same event, the completion of the production of something by one process is a different kind of event from the beginning of the processing of that thing by a second process. That is, the output of the first process is the input to the second process, not the beginning of the processing of this input by the second process.

And if the outputting of something from one process, and the inputting of that thing into a second process, are different kinds of events then, by definition, they can't ever be the same event. That is, they can't ever be different perspectives of the same event. Although, they can be immediately consecutive.

So the outputting of claim X from the first process can’t also be, in itself, the inputting of X into the process of thinking about X.

And, again, the outputting of X from the first process is by definition the entrance of X into our mind. That is, the completion of the cognitive process of comprehending, recalling, concluding or imagining-up a claim is by definition the entrance of that claim into our mind.

So the entrance of X into our mind via any cognitive process can't itself consist of us thinking about X. Although, the brain's speediness means that we can begin thinking about X within a fraction of a second of X entering our mind, and so it can seem that we were doing so upon X entering our mind.

So the entrance of X into our mind via any cognitive process must consist of us thinking X, given that there’s no other possibility.

Indeed, the above analysis regarding thinking about X doesn't similarly apply to thinking X. That analysis shows that the entrance of X into our mind can't consist of us thinking about X because thinking about X is a cognitive process that can only begin after the completion of the cognitive process by which X enters our mind. But thinking X is a specific thought, and a specific thought is not a cognitive process but the output of a cognitive process – which aligns with the entrance of X into our mind being the output of a cognitive process.

In the case of concluding X, this by definition consists of us thinking X, but comprehending, recalling or imagining-up X can seem to consist of us thinking about X. However, again, this is an illusion due to the brain's speediness.

It might be objected that although:

• the outputting of X from the first process can’t also be, in itself, the inputting of X into the process of thinking about X

• comprehending, recalling, concluding or imagining-up X involves, by definition, the entrance of X into our mind

the cognitive activity involved in comprehending, recalling, concluding or imagining-up X may actually involve not just the first process – the process that creates, recreates or retrieves X – but also the process of thinking about X.

That is, the first process and the process of thinking about X may actually be consecutive sub-processes of the process of comprehending, recalling, concluding or imagining-up X. And if the completion of this process involves the sub-process of thinking about X, then the entrance of X into our mind via any cognitive process consists of us thinking about X.

However, consider comprehending a claim, and consider that this is a cognitive task.

The performance of any kind of task – whether by our mind alone, or our mind and body, or a machine – is a whole process. That is, the beginning of the performance of a task is the beginning of a process, and the completion of the task is the completion of that process, with the goal of the task being the output of the process.

The performance of a task may involve the performance of sub-tasks, with such activities therefore being sub-processes. And that task may itself be a sub-task, with the performance of the task therefore itself being a sub-process. But the performance of a task is a whole process even if that process involves sub-processes or is a sub-process, or both.

So the performance of the cognitive task of comprehending a claim is a whole cognitive process.

Also, again, the completion of the cognitive process of comprehending claim X is by definition the entrance of X into our mind. And, by definition, X exists in our mind from the beginning of the process of thinking about X. So if the process of comprehending X involved both the first process and the process of thinking about X it would actually only involve the first moment of the latter process. That is, the beginning of the process of thinking about X would also be the completion of the process of comprehending X.

However, although a process can consist of two consecutive sub-processes, one process and just the beginning of another process don't together constitute a whole process. That is, the completion of a process is, by definition, a process ending, not beginning. Whereas the completion of a process involves outputting, the beginning of a process involves inputting.

So the completion of the process of comprehending X can't be the beginning of the process of thinking about X. Again, thinking X is the completion of a process.

So the process of comprehending X can't involve both the first process and the process of thinking about X, and therefore must be simply the first process. And the same logic applies to recalling, concluding or imagining-up a claim, which are also cognitive tasks, the performance of which is a whole cognitive process.

It might also be objected that although the outputting of something from one process, and the inputting of that thing into a second process, are different kinds of events, and so by definition can't ever be the same event, different events can be simultaneous, and so that outputting and inputting can be simultaneous. So it's possible for the process of thinking about claim X to begin upon X entering our mind.

It's indeed possible for the outputting of something from one process, and the inputting of that thing into a second process, to be simultaneous. For example, consider again the process of book manufacturing. As with any physical object, a book is subject to the forces of decay from the moment that it's formed. So although the completion of the production of a book by the process of book manufacturing is a different event from the beginning of the process of the book's decay – indeed, they're exactly opposite in nature – they're simultaneous.

However, given that the outputting of claim X from the first process is a different event from the inputting of X into the process of thinking about X, and is by definition in itself the entrance of X into our mind, which necessarily involves thinking X, we actually can't be thinking about X upon X entering our mind. That is, the outputting of X from the first process, and the inputting of X into the process of thinking about X, can't even be simultaneous – although, again, they can be immediately consecutive.

Indeed, given that the entrance of X into our mind is by definition the completion of the process of comprehending, recalling, concluding or imagining-up X, if that entrance consisted of the beginning of the process of thinking about X, then the beginning of the process of thinking about X would be the completion of the process of comprehending, recalling, concluding or imagining-up a claim – which, again, it can't be.

The conclusion that the entrance of X into our mind via any cognitive process must consist of us thinking X, together with the conclusion of the previous section that thinking X is always the mental state of believing X, however we come to be thinking X, means that the entrance of X into our mind via any cognitive process must always be the mental state of believing X.

And the analysis so far shows that our belief of X upon X entering our mind is due to merely that entrance, rather than the nature of the cognitive process by which X enters our mind. That is, that analysis shows that the entrance of X into our mind is the outputting of X from a cognitive process, and outputting can't also be inputting, and so the entrance of X into our mind can't itself be the inputting of X into the cognitive process of thinking about X, and so must consist of us thinking X, which is the mental state of believing X.

So the mere entrance of a claim into our mind, whether via comprehension, recollection, reason or imagination, causes us to believe it.

And this belief continues beyond the claim's entrance into our mind, due to the content of X being stored in our memory as an aspect of reality. Again, believing X doesn't necessarily involve currently thinking X.

Even if we begin thinking about X within a fraction of a second of X entering our mind, and the content of X therefore then exists in our mind as the content of X rather than as an aspect of reality, this isn't the end of our belief of X. Again, although we can think about X while not believing X, we can also think about X while believing X.

I call this theory of belief formation credulism, given its extreme claim about our credulity.

Again, credulism can seem obviously false – which can itself seem to disprove it. It can seem obviously contrary to both the concept and experience of comprehending, recalling or imagining-up a claim. Regarding reason, a conclusion is by definition a belief. But credulism states that our belief of this claim upon it entering our mind via our reason is due merely to that entrance, rather than being due to the claim being the product of our reason – which can seem obviously contrary to both the concept and experience of reasoning.

But consider, first, comprehending a claim.

Credulism and comprehending a claim

Consider our experience of comprehending a claim.

It can seem obvious from this experience that the entrance of a claim into our mind upon comprehending it doesn't cause us to believe it. For example, it can seem obvious from our experience of comprehending the claim London is the capital of France that the entrance of this claim into our mind just then didn't cause us to wrongly believe that London is the capital of France. Indeed, it can seem obvious that we instead just immediately thought/believed 'London isn't the capital of France'.

However, given that our thinking that London isn't the capital of France was a response to the comprehended claim, it must have actually occurred immediately after we comprehended that claim. So this immediate thought/belief actually doesn't in itself mean that we didn't believe the comprehended claim upon comprehending it – which we did, but that belief only lasted a fraction of a second.

The comprehended claim is obviously incompatible with the set of mutually compatible beliefs of ours that are relevant to the claim – which includes the beliefs A country's capital is located in that country, London is located in England, England and France are different countries, London is the capital of, and only of, England and the United Kingdom, Paris is located in France and Paris is the capital of France.

And the obviousness of this incompatibility means, by definition, that our awareness of it required very little thinking – as did our subsequent conclusion, based on that incompatibility, that London isn't the capital of France, and that the claim London is the capital of France is therefore false. Also, again, the brain functions on the timescale of milliseconds.

So our belief of the claim London is the capital of France only lasted a fraction of a second. And the extreme briefness of this belief means that we've no awareness of both it and our credulity. Hence our sense that the entrance of this claim into our mind upon comprehending it didn't cause us to believe it.

And the same applies to our comprehension of any claim that immediately seems false.

That includes the theory of credulism itself. Again, the fact that credulism can seem obviously false can itself seem to disprove it. But we in reality do believe it upon comprehending it. However, its apparent obvious falseness means that this belief only lasts the fraction of a second that it takes us to conclude that the mere entrance of a claim into our mind doesn't cause us to believe it.

The above analysis similarly applies to our comprehension of claims that immediately seem uncertain.

For example, if we hear someone declare that it'll rain on Friday next week this claim will likely immediately seem uncertain, given the uncertainty inherent in weather forecasts and the increasing probability of them being wrong the further in the future they refer to. But even if the claim does immediately seem uncertain we'll actually believe it upon comprehending it. However, this belief will only last the fraction of a second that it takes us to conclude that it's uncertain that it'll rain on Friday next week. And the extreme briefness of our belief means that we'll have no awareness of both it and our credulity.

We also comprehend claims that don't immediately seem false or uncertain, but which we didn't already believe. For example, if we don't hold any beliefs about David's dietary choices, and then hear someone say that he's vegetarian, we may not immediately think of a reason to disbelieve or even doubt this, especially if we don't know anything about David. If so, our belief of this claim upon comprehending it won't be replaced by a contrary belief within a fraction of a second of forming.

But although our belief of such a claim upon comprehending it can therefore last long enough for us to be able to be aware of both it and our credulity, we tend not to be even in such cases.

Being aware of our belief of claim X obviously involves thinking about our belief of X, which involves thinking about X. So, given the difference between thinking X and thinking about X, although thinking/believing X constitutes this belief being involved in our thinking, it doesn't in itself actually involve thinking about this belief. That is, thinking/believing X doesn't in itself actually involve being aware of our belief of X.

For example, although thinking/believing ‘David is vegetarian’ constitutes this belief being involved in our thinking, it doesn’t in itself actually involve thinking about this belief and thus being aware of it. That is, it involves, in itself, thinking about David, and him being vegetarian, without thinking about ourselves, and our belief that David is vegetarian.

Just as a claim can exist in our mind without us thinking about it, a belief can exist in our mind without us thinking about it, because we can think about the content of the belief without thinking about our belief of that content.

If we're thinking about our belief of claim X while apparently thinking/believing X then we're actually not at that moment thinking/believing X. Although, given the brain's speediness, we may think about our belief of X so soon before or after thinking/believing X that our thinking/believing X can seem to in itself involve thinking about this belief.

So when we comprehend a claim that doesn't immediately seem false or uncertain, but which we didn't already believe, we can't be aware of our belief of this claim upon comprehending it and whenever we're subsequently thinking/believing it.

Of course, we can think about, and thus be aware of, this belief when we're not thinking/believing the claim. But we tend not to, for several reasons.

First, our absence of uncertainty in the claim means that we tend not to be motivated to question, and thus think about, this belief. That is, our absence of uncertainty means that we tend to think about the content of the belief without thinking about our belief of that content.

Also, we’re usually too cognitively busy thinking about the content of our beliefs, including trying to make inferences from that content, to think about our belief of that content.

Our minimum waking cognitive activity involves continuously trying to work-out:

• what's happening in the immediate present, often including what's happening in other people’s minds

• why it's happening

• what's likely to happen in the immediate future, often including what's likely to happen in other people’s minds

• what should be our goals for the immediate future, taking into account our longer-term goals

• how best to act in the immediate future in order to achieve our immediate goals, often including how best to interact with other people.

This cognitive activity involves a constant stream of belief formation. And, again, it's just our minimum waking cognitive activity.

And our degree of cognitive busyness increases as the following factors increase:

• time pressure

• the quantity of information being processed

• the difficulty of the current cognitive task – which increases as stress, tiredness or distraction increases

• the number of cognitive tasks being performed in parallel.

We can sometimes be so cognitively busy that even when we comprehend a claim that would normally immediately seem false or uncertain our belief of it isn't replaced by a contrary belief within a fraction of a second of forming, and can last until our level of cognitive busyness has reduced to normal.

Also, the less important it is to us whether a believed claim is indeed true, the less likely we are to be motivated to question, and thus think about, our belief of it. And for many of our beliefs the actual truth of the believed claim isn't that important to us – as could be the case in the above example of our belief that David is vegetarian, after hearing someone state this.

So, for all of these reasons, even when our belief of a comprehended claim that we didn't already believe lasts long enough for us to be able to be aware of it, we tend not to be. And we of course can't be aware of our credulity in the formation of this belief if we aren't aware of the belief.

Also, even when we're aware of the belief, our absence of uncertainty means that we tend not to be motivated to question how it formed, and we're usually too cognitively busy to think about how it formed. Also, the less important it is to us whether the believed claim is indeed true, the less likely we are to be motivated to question how the belief formed.

Also, generally, the more time that has passed since the belief formed the less likely we are to think about this event. And even when we cease holding the belief, and then think about our past belief of this claim that we now consider false or uncertain, we won’t necessarily think about how it formed.

And we of course can't be aware of our credulity in the formation of the belief if we’re aware of the belief but don't think about how it formed.

And even if we do consider the formation of the belief – whether or not we still hold it – we may not remember that it formed upon us hearing or reading the claim concerned. And even if we do we can assume that the belief formed just after our comprehension of the claim, via our reason or intuition.

To summarise the conclusions of the analysis in this section so far, when we comprehend a claim that we didn't already believe, our resulting belief of it either only lasts a fraction of a second – if the claim immediately seems false or uncertain – and so we have no awareness of the belief, or it lasts long enough for us to be able to be aware of both it and our credulity, but even then we tend not to be aware of that credulity, or even the belief.

And in the case of comprehending a claim that we did already believe, we can of course attribute our belief of it upon doing so, and subsequently, to our prior belief of it.

We're therefore only occasionally aware of our credulity upon hearing or reading a claim. And so credulism can seem obviously contrary to our overall experience of comprehending claims, even though closer analysis reveals that it isn't.

Consider also our experience of other people comprehending claims.

We're by nature motivated to try to understand the world around us, which includes trying to understand other people. So we're by nature motivated to think about other people's beliefs, including how those beliefs formed. And this in turn means that we’re much more likely to be aware of other people’s credulity than they are, or than we are of our own credulity.

But if someone comprehends a claim that they didn’t already believe, and their resulting belief only lasts a fraction of a second, because the claim immediately seems to them false or uncertain, then not only will they not be aware of this belief, neither will we.

And even if the belief lasts long enough for us to be able to be aware of it, via what they say and do, we may not know or remember that it formed upon them comprehending the claim. And even if we do know and remember this we can assume that the belief formed just after their comprehension of the claim, via their reason or intuition.

And in the case of someone comprehending a claim that we think they already believed, we can of course attribute their belief of it upon them doing so, and subsequently, to their prior belief of it.

So although we're much more often aware of other people’s credulity upon hearing or reading claims than we are of our own, we're still only occasionally aware of the former. And so credulism can seem obviously contrary to our overall experience of other people comprehending claims, even though closer analysis reveals that it isn't.

It might be objected that it's both true by definition and obvious from experience that if we comprehend a claim that we didn't already believe, then our level of trust in the claim's source on the subject of the claim affects whether we believe the claim, contrary to credulism.

But, again, if we're aware that one of our beliefs, past or present, formed upon us hearing or reading the claim concerned, we can assume that it formed just after our comprehension of the claim, via our reason or intuition. So we can sometimes assume that the belief was the product of our consideration, just after comprehending the claim, of the trustworthiness of the claim’s source on the subject of the claim – whether or not such a consideration occurred, and, if it did, whether or not we recall it.

Also, if we comprehend a claim, X, that we didn’t already believe, and we distrust its source on the subject of X, then, given the brain's speediness, our resulting belief of X will only last the fraction of a second it takes us to conclude a contrary claim, such as It's uncertain whether X. And the extreme briefness of our belief of X means that we’ll have no awareness of it. So it can then seem that our distrust of X’s source on the subject of X prevented our belief of X.

It’s indeed true by definition that if we comprehend a claim that we didn't already believe, then our level of trust in the claim's source on the subject of the claim affects whether we believe the claim – but only after we comprehend/believe it. That is, it only affects whether we continue to believe the claim.

Although, there are two scenarios in which we comprehend a claim, X, that we didn’t already believe and our trust in X’s source on the subject of X leads to our belief of X in a way that isn't contrary to credulism.

The first is when that trust led us to pay attention to, or consult, that source, which then led to our comprehension/belief of X.

But such trust can even lead to our belief of X after we've comprehended X. That is, if after comprehending/believing X we cease believing X for some reason, we may subsequently either remember that we trust X’s source or develop such trust, and that trust may then lead us to conclude X.

Now consider the concept of comprehending a claim.

Credulism is indeed contrary to the current concept of comprehending a claim, according to which merely comprehending a claim, that we didn't already believe, doesn't involve us believing it. That is, the current concept involves the content of such a claim entering our mind as merely the content of a claim rather than as an aspect of reality.

But the current concept arose partly from the fact that, as I explained, we tend not to be aware of our credulity upon comprehending a claim. Also, any conception of comprehending a claim must, by definition, involve the claim entering our mind, and it has been wrongly assumed that this is all that comprehending a claim involves.

And there’s no reason to think that the concept of comprehending a claim can’t be revised to incorporate the idea that the mere entrance of a claim into our mind causes us to believe it. Indeed, as I've shown, there's reason to think that this concept must be revised in this way.

Note that although the revised concept of comprehending a claim includes the claim being believed, it's still about simply the claim entering our mind. That is, to refer to someone comprehending claim X, using this revised concept, is still to refer simply to X entering their mind via the process of comprehension. The change is that it's understood that their comprehending X involves believing X, because the entrance of a claim into our mind causes us to believe it.

Now consider recalling or imagining-up a claim.

Credulism and recalling or imagining-up a claim

Consider first our experience of recalling or imagining-up a claim.

It can seem obvious from this experience that the entrance of a claim into our mind upon being recalled or imagined-up doesn't cause us to believe it. But the analysis in the previous section similarly applies.

So when we recall or imagine-up a claim that we didn't already believe, our resulting belief of it either only lasts a fraction of a second – if the claim seems obviously false or uncertain – and so we have no awareness of it, or it lasts long enough for us to be able to be aware of both it and our credulity, but even then we tend not to be aware of that credulity, or even the belief. And in the case of recalling or imagining-up a claim that we did already believe, we can of course attribute our belief of it upon doing so, and subsequently, to our prior belief of it.

We're therefore only occasionally aware of our credulity upon recalling or imagining-up a claim. And so credulism can seem obviously contrary to our overall experience of recalling and imagining-up claims, even though closer analysis reveals that it isn't.

Regarding imaging-up a claim, this is either deliberate or spontaneous. That is, a claim enters our mind via our imagination because either we decided to generate a claim with our imagination or our imagination spontaneously generated the claim. For example, the claim David is vegetarian can enter our mind via our imagination because we decided to use our imagination to generate a claim about David, or because our imagination spontaneously generated this claim as we observed David eating a meat-free meal.

And in the case of deliberately imagining-up a claim, the fact that the entrance of the claim into our mind is immediately preceded by our intention to imagine-up a claim means that immediately after that entrance we'll be aware that the claim entered our mind just then as the product of our imagination. So the claim will seem obviously uncertain for at least that reason. That is, even if the claim doesn’t otherwise seem obviously uncertain, or seem obviously false, our belief of it will only last a fraction of a second, and so we’ll have no awareness of it.

Regarding recalling a claim, this is similarly either deliberate or unintentional. That is, we can recall a claim because either we made an effort to do so or something triggered the recollection.

And in the case of deliberately recalling a claim, the fact that the entrance of the claim into our mind is immediately preceded by our intention to recall it means that immediately after that entrance we'll be aware that the claim entered our mind just then as the product of our recollection. But that awareness doesn't in itself give us reason to be uncertain about it.

This is most obvious in the case of deliberately recalling an apparent fact – that is, a claim that we believed immediately before recalling it. Our awareness, immediately after the claim enters our mind, of why it did so obviously doesn't in itself give us reason to be uncertain about it.

Comprehending a claim is inherently deliberate. That is, it by nature involves a desire to obtain an understanding of the content of this claim that's being communicated to us via some medium that we're perceiving, which is normally voice or text.

And the fact that the entrance of the claim into our mind is immediately preceded by our intention to comprehend it means that immediately after that entrance we'll be aware that the claim entered our mind just then as the product of our comprehension. But that awareness also doesn't in itself give us reason to be uncertain about it.

As with deliberately recalling a claim, this is most obvious in the case of comprehending a claim that we already believe. Our awareness, immediately after the claim enters our mind, of why it did so obviously doesn't in itself give us reason to be uncertain about it.

Consider also our experience of other people recalling or imagining-up claims. The analysis in the previous section again similarly applies. So although we're much more often aware of other people’s credulity upon recalling or imagining-up claims than we are of our own, we're still only occasionally aware of the former. And so credulism can seem obviously contrary to our overall experience of other people recalling or imagining-up claims, even though closer analysis reveals that it isn't.

In the previous section I also addressed the potential objection that it seems both true by definition and obvious from experience that if we comprehend a claim that we didn't already believe, then our level of trust in the claim's source on the subject of the claim affects whether we believe the claim, contrary to credulism. And the analysis countering this objection similarly applies to the same objection regarding recalling a claim that we didn’t already believe.

Now consider the concepts of recalling and imagining-up a claim. Again, the analysis in the previous section similarly applies.

Credulism is indeed contrary to the current concepts of recalling and imagining-up a claim, according to which merely recalling or imagining-up a claim, that we didn't already believe, doesn't involve us believing it. That is, the current concepts involve the content of a such claim entering our mind as merely the content of a claim rather than as an aspect of reality.

But the current concepts arose partly from the fact that, as I explained, we tend not to be aware of our credulity upon recalling or imagining-up a claim. Also, any conception of recalling or imaging-up a claim must, by definition, involve the claim entering our mind, and it has been wrongly assumed that this is all that recalling or imaging-up a claim involves.

And there’s no reason to think that the concepts of recalling and imaging-up a claim can’t be revised to incorporate the idea that the mere entrance of a claim into our mind causes us to believe it. Indeed, as I've shown, there's reason to think that these concepts must be revised in this way.

Note that although the revised concepts of recalling and imagining-up a claim include the claim being believed, they're still about simply the claim entering our mind. That is, to refer to someone recalling or imagining-up claim X, using these revised concepts, is still to refer simply to X entering their mind via the process of recollection or imagination. The change is that it's understood that their recalling or imagining-up X involves believing X, because the entrance of a claim into our mind causes us to believe it.

I've said that organically thinking X is when our thinking X is either a recollection of an apparent fact or a conclusion. But in reality comprehending X, recalling X when we didn’t believe X immediately before doing so, and imagining-up X, also involve organically thinking X. In each case, by definition, X only enters our mind upon us thinking X, whereas deliberately thinking X, like a silent mantra, is by definition immediately preceded by us thinking about X.

Also, deliberately thinking X is always immediately preceded by organically thinking X, given that it's dependent on recalling this claim that we decided to think.

Now consider reason.

Credulism and reason

Again, a conclusion is by definition a belief. But credulism states that our belief of this claim upon it entering our mind via our reason is due merely to that entrance, rather than being due to the claim being the product of our reason – which can seem obviously contrary to both the concept and experience of reasoning.

But consider first the concept of reasoning.

Reasoning involves determining apparent logical implications of particular premises. For example, if we have to make a meal for David, and need to decide what to make, and we learn that he's vegetarian, then our reasoning will probably begin with these two premises:

David is vegetarian

Vegetarians don’t eat meat

and we'll then determine the logical implication:

David doesn’t eat meat

which we’ll then probably consider along with the premise:

I shouldn't make a meal containing meat for someone who doesn’t eat meat

to determine the logical implication:

I shouldn't make a meal containing meat for David.

Although, given the brain's speediness, and the obviousness of these two logical steps, it can seem to us that we simply went straight from considering that David is vegetarian to concluding that we shouldn't make a meal containing meat for him.

What follows logically from particular premises is of course independent of the truth of those premises and of the logical implications of them. For example, the claim All cats can fly is a logical implication of the premises All cats are green and All green things can fly even though all three claims are false.

So the process of determining a logical implication of particular premises isn't itself concerned with the truth of those premises or of the apparent logical implication of them. Therefore, although this process explains our belief that claim X is a logical implication of our premises, it doesn’t by itself explain our belief of X upon it entering our mind via this process.

Without knowledge of credulism, it might be thought that, immediately after X enters our mind, we consider our belief of the premises, and so conclude X. And this occurs within a fraction of a second of X entering our mind, given the brain's speediness and the obviousness of this conclusions – hence our sense that we believe X upon it entering our mind.

One fatal problem with this idea is that our concluding X would, by definition, involve a further logical step. If our belief of the product of the original logic step was dependent on a second logical step, then our belief of the product of the second logical step would likewise be dependent on a third logical step, and so on indefinitely. So our belief of X would be dependent on an infinite number of logical steps, and we'd therefore never be able to form beliefs via reason.

Of course, the other fatal problem with this idea that, given credulism, it would be unnecessary to conclude X, after X entered our mind, in order to believe X.

It might alternatively be thought, without knowledge of credulism, that our brain could be wired so that we automatically believe X upon X entering our mind as a logical implication of premises that we believe. But given credulism it's unnecessary for the brain to be wired in this way.

So our belief of X upon X entering our mind as a logical implication of our premises is due merely to that entrance.

Of course, when we form a conclusion our belief of our premises was necessary for us to use them as the basis of our reasoning. And our belief of the validity of our logic was obviously necessary for us to apply it to those premises. And the application of that logic to those premises is obviously what led to the entrance into our mind of the claim that we thereby believed.

But our belief of that claim upon it entering our mind isn't due to our belief of our premises and of the validity of our logic. That is, it isn't due to the claim being the product of our reason. It's due merely to the entrance of the claim into our mind upon being produced by our reason. So our reasoning leads to, but doesn't itself cause, our belief of the claim upon it entering our mind. In other words, although to conclude claim X is by definition to believe X, it isn't the case that belief X forms because we've concluded X – instead, we conclude/believe X because X entered our mind.

So although credulism is indeed contrary to the current concept of reasoning, it's that concept that's wrong. And there’s no reason to think that this concept can’t be revised to incorporate the idea that the mere entrance of a claim into our mind causes us to believe it. Indeed, as I've shown, there's reason to think that this concept must be revised in this way.

Now consider our experience of reasoning.

Again, it can seem obvious from our experience of forming a conclusion that our belief of this claim upon it entering our mind via our reason is due to it being the product of that reason.

But our experience of forming a conclusion would actually be the same whether this belief formation is due to the claim being the product of our reason or to the mere entrance of the claim into our mind upon being produced by that reason. That is, either way this experience would consist of us forming a belief of the claim upon it entering our mind via our reason.

So credulism actually isn't contrary to our experience of reasoning. When we form a conclusion, our sense that our belief of this claim upon it entering our mind is due to the claim being the product of our reason is actually merely our interpretation of this experience, based on the current concept of reasoning.

And the same applies to our experience of other people forming beliefs via reason: this experience would be the same whether this belief formation is due to the claim being the product of the person’s reason or to the mere entrance of the claim into their mind upon being produced by that reason.

It might be thought that credulism is definitely contrary to our experience of reasoning in some scenarios.

I've been saying that a claim that enters our mind via our reason is a conclusion. But although a conclusion is by definition a claim that has entered our mind via our reason, there are actually some occasions when a claim that enters our mind via our reason isn't a conclusion.

We sometimes generate a claim via logic that we believe is flawed or questionable. For example, we deliberately apply apparently flawed or questionable logic to particular premises in order to illustrate such logic. We also sometimes generate a claim via logic that’s based on premises that we either disbelieve or are uncertain about. For example, we determine apparent logical implications of premises that we either disbelieve or are uncertain about when undertaking hypothetical, 'What if...', reasoning.

In such cases, our belief about the validity or premises of the logic that we use to generate claim X means that the conclusion of our reasoning isn't simply X, but a claim of either the form X is an illogical [or possibly illogical] inference from the premises or the form X is a logical implication of the false [or uncertain] premises.

But in such cases X does first enter our mind by itself, upon being generated by such logic. Indeed, by definition, the cognitive process of generating the claim about X involves thinking about X, which involves X existing in our mind by itself. And credulism implies that we believe X upon it entering our mind by itself. However, the idea that we form belief X during such reasoning, despite our belief about the validity or premises of the logic that we used to generate X, can seem obviously contrary to our experience of such reasoning.

But given the brain's speediness, and our belief about the validity or premises of the logic that we used to generate X, our belief of X upon it entering our mind only lasts the fraction of a second that it takes us to conclude X is uncertain. And the extreme briefness of our belief of X means that we've no awareness of it.

Another potential objection

It might be objected that even if all of the analysis so far is sound there are still some scenarios in which the mere entrance of a claim into our mind doesn't cause us to believe it.

Consider a claim, Y, which concerns claim X, such as The claim 'David is vegetarian' is true or Amy said that David is vegetarian.

As I said in the previous section, by definition, the cognitive process of generating a claim about X with our reason involves thinking about X, which involves X existing in our mind by itself. And credulism implies that we believe X upon it entering our mind by itself. And the same applies to generating Y with our imagination.

But consider comprehending or recalling Y, which don't in themselves involve us generating this claim about X. Although the entrance of Y into our mind in each case involves us thinking/believing Y, thinking Y is thinking about X rather than thinking X. So it might be thought that the entrance of X into our mind when we comprehend or recall Y doesn't cause us to believe X.

But even in these two scenarios the entrance of X into our mind does cause us to believe X.

Given that X is part of Y we comprehend X as part of comprehending Y, and recall X as part of recalling Y. And so X enters our mind by itself during these processes too, which involves us thinking/believing X. Although, given the brain's speediness, Y can enter our mind within a fraction of a second of X doing so, and so these two mental events can seem to be the same event.

In the case of comprehending or recalling the claim The claim 'David is vegetarian' is false, we'll first believe the claim David is vegetarian, but then believe, within a fraction of a second, that this claim is false, and so then conclude, within a further fraction of a second, that David isn't vegetarian.

Even if X is the last part of Y – as in Amy said that David is vegetarian – comprehending X, after comprehending the preceding part of Y, isn't also comprehending Y, because comprehending Y is dependent on us then combining our comprehension of X with our comprehension of that preceding part. Although, again, our comprehension of Y will occur within a fraction of a second of our comprehension of X, and so these two mental events can seem to be the same event. And the same applies to recalling Y.

Even if we used our imagination to generate the claim Amy said that David is vegetarian not by first considering the claim David is vegetarian, but by creating the word string Amy said that David is vegetarian by generating each word in turn, the two claims wouldn't enter our mind simultaneously upon the last word, vegetarian, entering our mind.

That is, given that the claim David is vegetarian is part of the claim Amy said that David is vegetarian, our understanding that we'd generated the claim Amy said that David is vegetarian would be dependent on, and so preceded by, our understanding that we'd generated the claim David is vegetarian.

The logical necessity of credulism

The argument I presented for credulism actually has a highly counterintuitive conclusion in two ways. Given that that argument uses logic alone, it reveals not only that the mere entrance of a claim into our mind via any cognitive process causes us to believe it, but also that this is due not to the nature of the human brain but simply logical necessity.

The central tenet of the field of psychology today is that how the human mind works is due solely to the wiring and chemistry of the human brain, which are in turn determined by a person’s genes and experiences. But although our capacity for belief is obviously due to the nature of the human brain – as is our capacity for reason, emotion, language, perception, and so on – our extreme credulity isn't. Instead, that extreme credulity is simply a logically necessary aspect of a claim entering the mind – and not just the human mind, but the mind of any intelligent entity.

At the beginning of modern physics and astronomy, the use of mathematics, a form of logic, alone to reach conclusions about the natural world, rather than in conjunction with observational data, was controversial, and considered unscientific by critics. And yet in physics and astronomy today this is accepted as a legitimate way to further our understanding of the natural world. Indeed, much of theoretical physics is the use of mathematics, and non-mathematical logic, alone to develop and refine theories.

And the logical necessity of credulism shows that using logic alone is a legitimate way to further our understanding of human psychology. Indeed, that logical necessity means that using logic alone is the only way of fully understanding belief formation. That is, although we could find observational evidence for credulism by studying brain activity, no amount of such evidence could show the logical necessity of credulism.

I explained in the previous article that the idea that the formation of belief X is us assessing that claim X is true is fatally logically flawed, and that that assessment is instead based on that belief. And at the end of that article I raised an alternative possible answer to the question of how beliefs form. The formation of belief X is us concluding X – however brief, irrational, emotional, trusting or intuitive our reasoning is. As I explained, even the formation of beliefs about our surroundings or body via sensory perception requires reason.

But the analysis in this article shows that this answer is also fatally logically flawed.

The idea that the formation of belief X is us concluding X is the idea that the formation of belief X is due to claim X being the product of our reasoning. But, by logical necessity, the formation of belief X upon X entering our mind via our reason is due merely to that entrance. So, by logical necessity, although to conclude X is by definition to believe X, it isn't the case that belief X forms because we've concluded X – instead, we conclude/believe X because X entered our mind.

Plus, also by logical necessity, the entrance of X into our mind via any other cognitive process – whether comprehension, recollection or imagination – also causes us to believe it.

Vindicating Spinoza

Again, this theory of belief formation was first proposed by the prominent 17th-century philosopher Baruch Spinoza, in his posthumous 1677 book Ethics.

However, the argument for credulism that I've presented in this article isn’t actually Spinoza’s, but was developed by me. Spinoza, whose writings aren't known for being a model of clarity, used an obscure philosophical argument. It at one point refers to the geometry of a triangle, and at another point refers to an imaginary winged horse. I'm not a philosopher and am unable to follow it. But even a philosopher I contacted who's an expert on Spinoza's philosophy admitted that they didn't understand the argument, and also weren't aware of any sources which presented an explanation of it.

The obscurity of Spinoza’s argument, and its highly counterintuitive conclusion, together explain why support for his theory has been very low since it was published.

The most prominent modern writing on the theory is a 1991 paper called ‘How Mental Systems Believe’, by Harvard psychology professor Daniel Gilbert, one of the relatively few academics who support the theory. It catalogues apparent supporting observational and experimental evidence, which I’ll review in an upcoming article in this series along with more recent apparent supporting evidence. An example of the latter, which I mentioned earlier, is a 2022 paper by neuroscientists at Harvard, Princeton and Dartmouth universities that presents brain scan evidence which seems to support the theory.

However, Gilbert's paper misrepresents Spinoza’s theory in one significant way. It only refers to the theory as applying to the entrance of claims into our mind via comprehension.

This is also true of most other papers and books which refer to the theory. And in most cases it seems likely that this is because the author based their understanding of the theory on either Gilbert’s paper or a paper or book by someone who based their understanding of the theory on Gilbert's paper. For example, the 2018 book Belief, by psychology professor James Alcock, another supporter of the theory, misrepresents the theory in this way, citing Gilbert’s paper.

In addition to presenting apparent supporting evidence for Spinoza’s theory, albeit only in relation to our comprehension of claims, Gilbert presents a speculative evolutionary explanation for this extreme credulity. He doesn't even briefly mention Spinoza's own argument – presumably because he was unable to follow it too.

He begins his argument with the premise that animals evolved to immediately believe what they perceive, and only question their perceptions afterwards, if at all. He argues that animals evolved to form beliefs via perception in this way because they often need to react quickly to their surroundings, and the content of their perceptions are a sufficiently reliable representation of their surroundings. He then says that perception is therefore 'quintessentially Spinozan'.

He next suggests that cognition could have evolved out of perception, given that both involve mental representations. That is, in the case of perception the mental representations are the content of our perceptions, and in the case of cognition they’re the content of claims involved in our cognition. Also, both the perceptual process and the process of comprehending claims involve processing sensory data to create the mental representation, given that the latter process includes our perception of the medium used to communicate a claim – which, again, is normally voice or text.

He then argues that the Spinozan nature of perception could therefore have been inherited by the process of comprehending claims. That is, just as we immediately believe what we perceive, we immediately believe what claims we comprehend.

He also points-out that most people communicate true claims most of the time, quoting philosophy professor Daniel Dennett: ‘The faculty of communication would not gain ground in evolution unless it was by and large the faculty of transmitting true beliefs’. So, Gilbert suggests, just as the general reliability of our perceptions allows us to immediately believe what we perceive, and thereby react quickly to our surroundings, so the general reliability of the claims that people communicate allows us to immediately believe what claims we comprehend, and thereby react quickly to those claims.

But there are three significant problems with Gilbert's argument.

First, the idea that perception is 'quintessentially Spinozan' is the idea that the perceptual process directly produces beliefs about our surroundings or body, before we've undertaken any reasoning about the content of our perceptions. But, again, as I explained in the previous article, even the formation of beliefs about our surroundings or body via sensory perception requires reason. Although, such reasoning is often so basic and therefore brief that the resulting belief can seem to be a direct product of the perceptual process.

A second problem is that the idea that perception is 'quintessentially Spinozan' actually contradicts the idea that cognition evolved out of perception. That is, belief is the quintessential cognitive state, and so the idea that the capacity for belief pre-dated the evolution of cognition is nonsensical.

A third problem is that the argument that I've presented in this article shows that our belief of claims upon comprehending them is simply a logically necessary aspect of such claims entering our mind, and so isn't due to the way that our brains evolved.

The cognitive scientist Hugo Mercier presents in his 2020 book Not Born Yesterday, on our degree of gullibility, another objection to the idea that Spinozan comprehension of claims could have evolved. Given the biological value of taking advantage of credulity by lying, the greater the credulity in our species, the greater the evolutionary pressure towards lying, and so, eventually, the greater the frequency of lying, and so the greater the evolutionary pressure away from credulity, given that believing false claims tends to be biologically costly.

Mercier uses this point to argue against not just the evolution of Spinozan comprehension of claims, but Spinozan comprehension of claims itself. His reasoning therefore doesn't take into account the possibility that such extreme credulity is simply logically inevitable, and that any biological cost of it is therefore irrelevant to whether it occurs. That is, his reasoning is based on the central tenet of the field of psychology today that how the human mind works is due solely to the nature of the human brain, which is the product of evolution.

But it might be thought that the logical inevitability of us believing every claim we comprehend should have created a very strong evolutionary pressure towards lying whenever it seems that we'd benefit from the lie being believed. And it might then be thought that the fact that the average person is far from being this dishonest implies that the logical necessity of credulism can't be true. However, this logical necessity actually doesn't imply the evolution of such an extreme level of dishonesty.

Although lying can initially benefit the liar, it also involves the high risk of the significant cost of reputational damage from the deception being found-out.

Although we don't always become aware that we've been lied to, we often do, however long it takes. Also, the more often someone lies the more likely that at least some of their deception will be found-out. And when we become aware that we've been lied to we tend to be strongly motivated to tell others about it, whether to complain or warn or both.

And knowledge of the deception will tend to significantly reduce how trusted the liar is to be honest, which will in turn tend to significantly reduce how moral in general they're perceived to be. And such significant reputational damage will tend to significantly negatively impact their life, whether or not they're aware of every specific way in which it does, or are even aware of the reputational damage, or to whom their deception has become known.

This reputational damage will have had especially negative implications for our hunter-gatherer ancestors, living in groups of only a few dozen, given the likelihood that all, or at least almost all, of those the liar interacted with in their daily life would have quickly learned of their lie.

Of course, our distrust of a liar can't prevent our belief of claims they subsequently make, given the logical necessity of such belief, but it leads to such beliefs being replaced within a fraction of a second by the belief that the content of the claim is uncertain, or even that it's unlikely, or false, depending on our level of distrust.

In the case of lies involving claims that immediately seem false, our awareness of the deception will therefore occur immediately – unless we conclude that person making the claim unintentionally made a false claim – and so the lie would be all cost and no benefit to the liar.

So, despite the logical inevitability of us believing every claim we comprehend, this significant downside to lying prevented the evolution of us universally lying whenever it seems that we'd benefit from the lie being believed.

Mercier, in arguing against Spinozan comprehension of claims, argues that we're instead 'endowed with sophisticated cognitive mechanisms of open vigilance'. He defines open vigilance as being open to the possibility that comprehended claims are true while also being vigilant against falsehoods.

It might be thought that, despite the logical necessity of credulism, such cognitive mechanisms could still exist, but would be, ironically, at least partly due to, rather than evidence against, our extreme credulity. That is, it might be thought that the logical inevitability of this extreme credulity, and the biological cost of believing false claims, creates an evolutionary pressure towards the development of cognitive mechanisms that increase our chances of quickly identifying false comprehended claims, and could even completely counteract our extreme credulity.

But consider a cognitive mechanism which, upon us comprehending a claim we didn't believe immediately beforehand, always triggered our assessment of its truth. Such a cognitive mechanism would be redundant.

For example, consider comprehending the claim David is vegetarian when we didn't believe it immediately beforehand. Our resulting belief that David is vegetarian means that David being vegetarian would exist in our mind as an aspect of reality upon us, in the next moment, assessing whether David is vegetarian. So we’d simply immediately assess that David is vegetarian, and that the claim David is vegetarian is therefore true.

It might be objected that when we believe claim X, but decide to assess whether X is true, we actually don’t simply immediately conclude X, and then conclude that X is true, but instead assess the basis of our belief. However, when we decide to assess whether X is true, as opposed to this assessment being automatically triggered by our comprehension of X if we didn't believe X immediately beforehand, this decision must be motivated by at least some degree of doubt about X, however small, and that doubt motivates our assessment of the basis of our belief.

This hypothetical cognitive mechanism could instead always trigger our assessment of the claim's truth not immediately after we comprehend it, but after a short delay, to give at least some time for reasons to doubt or disbelieve the claim to occur to us. However, any reason to doubt or disbelieve the claim that occurs to us immediately after comprehending the claim would have done so without the cognitive mechanism, and would make that mechanism redundant.

Another possibility is a cognitive mechanism which, upon us comprehending claim X when we didn't believe X immediately beforehand, always immediately replaces our belief of X with the belief that X is merely a possibility. But there actually wouldn't be any evolutionary advantage to such a cognitive mechanism.

When we consider that our comprehension of any claim necessarily involves believing it, we naturally tend to focus on the disturbing implication that we inevitably believe any false claim upon hearing or reading it. But a comprehended claim may of course be true.

Even a lie can be true. A lie is claiming X when we believe that X is false, and we intend to deceive. So even if X is actually true, our claiming X is a lie.

And although believing false claims tends to be biologically costly, so does ceasing to believe true claims. And there’s no reason to think that the former will on average be more biologically costly than the latter regarding the claims that we comprehend. Hence, a cognitive mechanism that always ended our belief of a comprehended claim immediately after we comprehended it, when we didn't believe it immediately beforehand, actually wouldn't be evolutionarily advantageous overall.

Also, if the evolution of communication was indeed dependent on most communicated claims being true, then such a cognitive mechanism would be evolutionarily disadvantageous overall. Also, our belief of false comprehended claims that immediately seem false, or at least uncertain, will only last a fraction of a second even without this mechanism.

It might be objected that, in the event of such a cognitive mechanism ending our belief of a true comprehended claim, our belief of it could form again soon afterwards – and, in the case of a true claim that immediately seems true, this would occur within a fraction of second. So, given the biological cost of believing all false comprehended claims, there actually would be an evolutionary advantage to this cognitive mechanism.

But it’s equally true that, in the event of such a cognitive mechanism ending our belief of a false comprehended claim, our belief of it could form again soon afterwards – and, in the case of a false claim that immediately seems true, this could occur within a fraction of second. So this objection actually doesn't show an evolutionary advantage to this cognitive mechanism.

Another possibility is a cognitive mechanism which, upon us comprehending claim X when we didn't believe X immediately beforehand, always immediately triggers a non-specific doubt about X. Such a doubt would be experienced as a vague intuitive doubt. But the analysis regarding the previous hypothetical cognitive mechanism would similarly apply to this one, and so there wouldn't be any evolutionary advantage to it either.

In sum, cognitive mechanisms specifically for counteracting our credulous comprehension of claims, even partially, couldn't have evolved. And the above analysis similarly applies to the idea of cognitive mechanisms evolving specifically for counteracting our belief of claims entering our mind via our imagination or recollection.

In the case of imagination, if we use this faculty to generate a random claim, then such a claim is of course much more likely to be false than true. But, again, when we deliberately imagine-up a claim our belief of it only lasts a fraction of a second. And when our imagination spontaneously generates a claim – such as the claim David is vegetarian upon us observing David eating a meat-free meal – there's no reason to think that such a claim will be more likely to be false than true, especially given that its generation is triggered by what we at least believe to be true.

Regarding Gilbert's argument, it is, in sum, fatally flawed, and so fails to support Spinoza's theory in relation to claims entering our mind via comprehension, in addition to not even attempting to support the theory in relation to claims entering our mind via the other three possible cognitive processes.

But the argument that I've presented in this article doesn't just support Spinoza's theory, and in full, it proves it, by showing that this extreme credulity is a logically necessary aspect of a claim entering the mind. Although, in addition to this theoretical proof, this extreme credulity must of course also be confirmed experimentally.

Credulism and supposed intuitive belief

The idea of intuitive belief originated to explain beliefs that don't seem to form via reason. A supposed intuitive belief instead forms via some nebulous instinct. For example, we may believe that someone is lying, but not be aware of why we believe this, and so conclude that this belief is an intuition.

But such beliefs actually often do form via reason. Again, the brain functions on the timescale of milliseconds. And such beliefs can form via reasoning that's simply so short, and therefore brief, that we're unaware of it. In the above example, there may have been something in the person's body language, including their facial expression, that led us to conclude, reasonably or not, that they're lying, but this reasoning occurred within a fraction of a second and so we're unaware of it.

Also, as I said earlier, when we're aware that a belief formed upon us comprehending, recalling or imagining-up the claim concerned we can assume that the belief formed via our reason just after the claim entered our mind. But if we try and fail to think of a plausible chain of reasoning that could have led to the belief we often attribute it to intuition.

In the example, the idea that the person is lying could instead enter our mind via our imagination. And if we're aware that our belief that they're lying formed upon this idea entering our mind via our imagination we can assume that this belief formed via our reason just after this idea entered our mind. But if we try and fail to think of a plausible chain of reasoning that could have led to this belief we may attribute it to intuition.

So the idea of intuitive belief originated partly due to ignorance of credulism – and also helps maintain that ignorance.

Of course, when we're aware that a belief formed when the claim concerned entered our mind via our reason, ignorance of credulism just means that we'll wrongly think that this belief formation was due to the claim being the product of that reason.

Why credulism matters

The fact that our belief of a claim upon it entering our mind via our reason isn’t due, as currently understood, to the claim being the product of that reason, and is instead due to merely that entrance, doesn’t itself actually have significant implications for human psychology, and therefore also for our lives. Although, knowledge of how beliefs form via reason matters at least in the sense that knowing the truth about the world, including our psychology, matters intellectually.

However, the fact that merely comprehending, recalling or imagining-up a claim involves believing it, contrary to the current understanding, and that this belief can last for any length of time, obviously does have significant implications for human psychology, and therefore also for our lives.

Regarding such beliefs that we attribute to intuition, the fact that they actually don't arise via some nebulous instinct means that we should be a lot more questioning of them. But there's a much more significant implication of credulism.

In the next article I’ll present a second highly counterintuitive theory about the psychology of belief – the theory that there aren't degrees of belief, that belief is certainty, and that this is so by logical necessity too. And in subsequent articles I’ll show that credulism and the certainty of belief together explain cognitive biases.

Since the early 1970s, researchers have discovered a large number of these universal biases in human cognition. A Wikipedia page currently lists around two hundred of them, and the list keeps growing.

For example, confirmation bias is our cognitive bias towards interpreting, seeking and recalling information in a way that confirms, or helps to confirm, what we currently believe. And availability bias is our cognitive bias towards making a judgement on the basis of whichever relevant information happens to be most readily available to our mind, instead of suspending judgement until we’ve checked for other relevant information.

Cognitive biases underlie our consistent reasoning errors. These include our consistent mathematical errors, which in turn includes our consistent statistical errors. And they even include those errors which occur when our reason is affected by our emotions. Cognitive biases therefore account for most of human irrationality.

There are many theories about why cognitive biases exist. Some only concern a single bias, whereas others try to unify several biases with a single explanation. All involve the cognitive bias or biases being due to the nature of the human brain.

But all cognitive biases are simply manifestations of credulism and the certainty of belief, and so are simply logically necessary aspects of cognition, for any intelligent entity.

The idea that our cognitive bias to do X is logically necessary isn’t the idea that doing X is logically inevitable, but merely that our bias to do X is logically inevitable. Whether we ultimately do X is dependent on our nature and the nature of our situation. So a particular person not doing X in a particular situation doesn't disprove the idea that we've a logically inevitable bias to do X.

Credulism and the certainty of belief therefore together provide a unifying explanation for all cognitive biases, and thus for most of human irrationality.

Next article: The Impossibility of Degrees of Belief

How Belief Works