How Belief Works

Chapter 4: Belief Is Certainty​

Why there are no degrees of belief​

It’s generally thought, including within psychology and philosophy, that we can have different degrees, or strengths, of belief. As I mentioned in chapter 1, it can seem obvious, for example, that our belief that we’re eating an apple is stronger than our belief that it’ll rain tomorrow.

However, I discovered that this supposed truism is logically flawed in three ways, and that belief is, by logical necessity, certainty. And closer analysis of our experience of belief reveals that this conclusion actually isn't contrary to that experience.

The certainty of belief isn't just of intellectual interest, but has profound implications for human psychology and therefore our lives.

The supposed relationship between confidence and belief

We can have different levels of confidence in a claim, with the maximum being certainty. And it’s currently thought that our confidence in a claim constitutes our belief of it if that confidence is above a particular sub-certain level, which is perhaps most commonly thought to be half-way between zero confidence and certainty:

Hence the current understanding that we can have different degrees, or strengths, of belief.

It doesn’t matter, for the argument presented in this chapter, what the sub-certain threshold for belief is considered to be – some people think that it’s much higher than 50% confidence, while others think that any level of confidence above zero constitutes belief, however weak.

Now consider what our confidence in a claim is, and how it arises.

The nature and origin of confidence

Our confidence in a claim is a feeling about whether the claim is true. Therefore a particular level of confidence in a claim must arise from a particular conclusion about whether the claim is true, however brief or unsound our reasoning – even if that reasoning simply involves uncritically accepting the word of someone else. But what is the nature of that conclusion?

The only logical possibility is that our particular level of confidence in a claim arises from our conclusion that the claim has a particular probability of being true. For example, our high but sub-certain confidence in the claim There are six eggs in the fridge, regarding a particular fridge, arises from our conclusion that it's highly likely that this claim is true. Sometimes we think more precisely about both the probability of a claim being true and our resulting feeling of confidence in the claim. For example, we may conclude that there's a 90% probability of the above claim being true, and that we therefore have 90% confidence in it.

Our certainty in a claim therefore arises from our conclusion that it’s certain that the claim is true. That is, the first reference to certainty in the previous sentence concerns the highest possible level of confidence, which is a mental state, whereas the second reference concerns the highest possible level of probability, which is a mathematical concept.

Our feeling of confidence in a claim can sometimes seem to be the product of not reasoning but our intuition – our supposed ability to understand something instinctively, and therefore without using reasoning. However, our ‘intuition’ is actually just reasoning that’s so brief, and therefore quick, that we're subsequently only aware of its outcome, given the speediness of the brain, which works on the timescale of milliseconds.

To be clear, the confidence that arises from our conclusion that claim X has a particular probability of being true is indeed a particular level of confidence in X, as opposed to confidence in the claim X is true. As I explained in chapter 3, the claims X and X is true are different claims. Therefore a feeling of confidence in the claim X is true must arise from a conclusion about the probability that this claim, not X, is true. Therefore our confidence in X isn't confidence in the truth of X, contrary to what's usually written in academic papers and books. For example, the philosopher Franz Huber begins his introduction to the 2009 philosophical anthology 'Degrees of Belief' as follows: 'Degrees of belief are familiar to all of us. Our confidence in the truth of some propositions is higher than our confidence in the truth of other propositions.'

Likewise, the confidence that arises from our conclusion that claim X has a particular probability of being true isn’t a particular level of confidence in this conclusion. That is, our confidence in this conclusion must arise from a subsequent conclusion about the probability that the first conclusion, not X, is true.

Also note that the wordings ‘our confidence in the claim There are six eggs in the fridgeand ‘our confidence that there are six eggs in the fridge’ are identical in meaning, because both are referring to our confidence in the claim that there are six eggs in the fridge. Likewise, the wordings ‘our belief of the claim There are six eggs in the fridgeand 'our belief that there are six eggs in the fridge’ are identical in meaning – if they’re both referring to belief in the sense of the psychological state.

To recap the two critical points of this section:

    • Our confidence in a claim is a feeling about whether it’s true.

    • Our particular level of confidence in claim X arises from our conclusion that X has a particular probability of being true.

Now consider how we form that conclusion.

The origin of our conclusion about the probability of a claim being true

As I explained in chapter 3:

    • The difference between the claims X and X is true means that our belief of them are different beliefs.

    • The definition of truth means that our assessment that X is a true claim must be based on our belief of X. For example, our assessment that the claim There are six eggs in the fridge is a true claim must be based on our belief that there are six eggs in the fridge.

Likewise, the definition of truth means that our assessment that X has a particular probability of being a true claim must be based on our belief that the content of X has that probability. For example, our assessment that it’s highly likely that the claim There are six eggs in the fridge is a true claim must be based on our belief that it’s highly likely that there are six eggs in the fridge.

As I also explained in chapter 3, there’s one type of occasion when our belief of X can be due to our conclusion that X is true. If we hear or read a claim, X, that we don’t already believe, and it’s made by someone who we think won’t be wrong on the subject of X, then we'll conclude that X must be a true claim, and therefore then conclude X. As I explained, our conclusion that X is true doesn’t actually constitute an assessment that X is true, given that it’s the product not of a comparison of X and reality, but of a consideration of the source of X.

Likewise, our belief that the content of X has a particular probability can be due to our conclusion that X has that probability of being a true claim. That is, if we hear or read X, and we don’t already have a belief about the probability of the content of X, and X is made by someone who we think has a particular probability of being correct on the subject of X, then we'll conclude that X must have that probability of being a true claim, and then conclude that the content of X has that probability. And our conclusion that X has a particular probability of being true doesn’t actually constitute an assessment that X has that probability of being true, given that it’s the product not of a comparison of X and reality, but of a consideration of the source of X.

Whether our belief that X has a particular probability of being a true claim forms after or before the formation of our belief that the content of X has that probability, they can seem to be the same event given both the speediness of the brain and our tendency to conflate the claims X and X is true.

The true relationship between confidence and belief

To recap the critical points so far:

    • Our particular level of confidence in claim X arises from our conclusion that X has a particular probability of being a true claim.

    • And this conclusion is based on either:

      • our belief that the content of X has that probability – scenario 1:

or

    • our belief that the source of X has that probability of being correct on the subject of X – scenario 2:

    • In the case of scenario 2, our conclusion that X has a particular probability of being a true claim is followed by our conclusion that the content of X has that probability.

Now consider the idea of degrees of belief. Again, according to this idea, our confidence in X constitutes our belief of X if that confidence is above a particular sub-certain level – perhaps most commonly thought to be half-way between zero confidence and certainty – with such levels of confidence constituting different degrees, or strengths, of belief. The analysis so far shows that there are three flaws in this idea.

First, whereas our confidence in X is a feeling about whether X is a true claim, our belief of X isn’t itself concerned with the truth of X, as I explained in chapter 3.

Second, in either scenario, if our feeling of confidence in a claim constitutes belief, then the formation of this belief would be dependent on the formation of the two beliefs which led to this feeling of confidence, and those beliefs would themselves involve feelings of confidence, which would therefore in turn be dependent on the formation of other beliefs, and so on indefinitely. Belief formation would therefore be impossible, and yet we easily, and constantly, form beliefs.

Third, given that our confidence in X is a feeling – about whether X is a true claimit's an emotional mental state, whereas belief isn't. That is, belief is, as I defined in chapter 2, an understanding of part of reality, and an understanding is, in itself, a purely intellectual mental state, and therefore not itself emotional. Of course, the content of a belief, and implications of that content, may arouse particular emotions, and such emotions may, given the speediness of the brain, arise within an unnoticeable fraction of a second of the formation of the belief.

These points show that, contrary to the idea of degrees of belief, confidence and belief are actually two different mental states, and so no level of confidence in X, even certainty, itself constitutes our belief of X.

It might be thought that our belief of X must therefore be a separate mental state that's based on our confidence in X, if that confidence is above a particular sub-certain level. That is, the mental state of sub-certain belief doesn't itself involve sub-certain confidence, and is instead based on such confidence. However, such a link still implies an indefinite, and therefore impossible, preceding chain of belief formation.

To understand the true relationship between confidence and belief, consider first our sub-certain confidence in a claim. In the example of our high but sub-certain confidence in the claim There are six eggs in the fridge, this arises from our conclusion that it's highly likely that this claim is a true claim. And in scenario 1 this conclusion is based on our belief that it's highly likely that there are six eggs in the fridge:

In scenario 2 this conclusion is instead the basis of our belief that it's highly likely that there are six eggs in the fridge, and is based on our belief that it's highly likely that the source of this claim will be correct about the existence of six eggs in the fridge:

Therefore what we believe, in either scenario, regarding the subject of the claim There are six eggs in the fridge is actually the different, albeit close in meaning, claim It's highly likely that there are six eggs in the fridge.

To be clear, even this belief doesn't constitute our sub-certain belief of the claim There are six eggs in the fridge. In addition to the above points about why confidence doesn't constitute belief, the difference between the claims There are six eggs in the fridge and It's highly likely that there are six eggs in the fridge means that our belief of them are, by definition, different beliefs.

So, if we have sub-certain confidence in claim X then what we believe regarding the subject of X is actually a different claim: X has sub-certain probability P. However, that confidence and belief can, given the speediness of the brain, form within an unnoticeable fraction of a second of each other, and so these two events can seem to be part of the same event. And we can subsequently switch between these two mental states so quickly that they can continue to seem to be part of the same mental state. And it can therefore seem that our belief that X has sub-certain probability P is our sub-certain belief of X.

To be clear, our belief of the claim X has sub-certain probability P doesn’t even involve a feeling of confidence in this claim that’s separate from our confidence in X, for the same three reasons why our confidence in X doesn't involve belief of X. And likewise for our belief about the probability that X is a true claim. Not only is the confidence that arises from this conclusion a separate mental state, it is, as I explained, confidence not in this conclusion, but in X. That is, in order for us to feel a level of confidence in this conclusion we need to subsequently assess the probability that this claim, not X, is true.

Now consider our certainty in a claim. Again, the terms certainty and certain can refer to either the highest possible level of confidence, which is a mental state – as in ‘His certainty that ...’ and ‘She’s certain that ...’ – or the highest possible level of probability, which is a mathematical concept – as in ‘It’s a certainty’ and ‘It’s certain’.

Our certainty in X – confidence – arises from our conclusion that it’s certain – probability – that X is a true claim. And that conclusion is based on either our belief that the content of X is certain – scenario 1 – or our belief that it's certain that the source of X will be be correct on the subject of X – scenario 2. In scenario 2, our conclusion that it’s certain that X is a true claim is followed by our belief that the content of X is certain.

For example, in scenario 1 our certainty in the claim There are six eggs in the fridge arises from our conclusion that it’s certain that this claim is a true claim, and this conclusion is based on our belief that it’s certain that there are six eggs in the fridge:

And in scenario 2 this conclusion is instead the basis of our belief that it's certain that there are six eggs in the fridge, and is based on our belief that it's certain that the source of this claim will be correct about the existence of six eggs in the fridge:

Contrary to what may be assumed, our belief that it’s certain that there are six eggs in the fridge isn't itself the belief that there are six eggs in the fridge. The former is belief of the claim It’s certain that there are six eggs in the fridge, whereas the latter is belief of the claim There are six eggs in the fridge. And whereas the first claim states a probability for the existence of six eggs in the fridge, the second doesn’t – it simply states that there are six eggs in the fridge. In scenario 1 our belief of the first claim is based on our belief of the second – that is, our belief that the probability of there being six eggs in the fridge is 100% is based on our belief that there are six eggs in the fridge. And in scenario 2 it's the reverse.

Therefore if we have certainty in X, our belief that the content of X is certain isn't itself our belief of X, in either scenario. Instead, either the former is based on the latter – in scenario 1 – or the reverse – in scenario 2. But these two beliefs can seem to be the same belief because they each follow so obviously from the other that we can fail to notice the basic logical step separating them.

So, unlike with any sub-certain level of confidence in X, our certainty in X means that we believe X, as is currently understood. Although, as with sub-certain confidence, that confidence and belief are different mental states, contrary to the current understanding. However, they can, given the speediness of the brain, form within an unnoticeable fraction of a second of each other, and so these two events can seem to be part of the same event. And we can subsequently switch between these two mental states so quickly that they can continue to seem to be part of the same mental state. And it can therefore seem that our belief that the content of X is certain is our belief of X with certainty.

Also as with sub-certain confidence, our belief of the claims X, X is certain, and It's certain that X is a true claim don’t even involve a feeling of confidence in these claims that's separate from our certainty in X.

However, whatever our level of confidence in a claim, there’s actually still a sense in which our belief regarding the subject of the claim involves, in itself, a level of confidence in the believed claim. Doubt about a claim is, like our confidence in a claim, a feeling about whether the claim is true. Therefore the logic of the above analysis about the relationship between confidence and belief similarly applies to the relationship between doubt and belief. Therefore any doubt about a claim is a different mental state from our belief regarding the subject of the claim. That is, the latter mental state doesn't itself involve a feeling of doubt. And certainty is often referred to as an absence of doubt. Therefore whatever our level of confidence in a claim, our belief regarding the subject of the claim involves, in itself, certainty in the believed claim in the sense of an absence of doubt.

To recap the critical points of this section so far:

    • Confidence and belief are two different mental states, and so no level of confidence in X, even certainty, itself constitutes our belief of X.

    • If our feeling of confidence in claim X is certainty, then we believe X.

    • If our feeling of confidence in X is sub-certain, then what we believe regarding the subject of X isn’t X, but a claim of the form X has sub-certain probability P.

    • Whatever our level of confidence in X, our belief regarding the subject of X at least involves, in itself, certainty in the sense of an absence of doubt, as opposed to a feeling of certainty.

And if the only level of confidence in X that’s associated with our belief of X is certainty, then we don’t have different degrees, or strengths, of belief, contrary to the general understanding, even within psychology and philosophy. Also, even our feeling of certainty in X isn’t itself our belief of X, although that belief is, in itself, at least certainty in the sense of an absence of doubt. I'll explain shortly why the certainty of belief has profound implications for human psychology and therefore our lives.

The certainty of belief is counterintuitive in another way. The above theoretical argument consists solely of conceptual analysis – that is, it doesn't at any point depend on observational evidence. And the argument is a continuous chain of logical implication. It therefore shows not only that belief is certainty, but also that this is so not because of the nature of the human brain, but simply by logical necessity.

As I mentioned in chapter 1, the central tenet of the field of psychology today is that how the human mind works is solely due to the wiring and chemistry of the human brain, which is in turn determined by a person’s genes and experiences. Therefore all mainstream theories about any aspect of the functioning of the human mind involve that aspect being due, in some way, to the nature of the human brain. However, although our capacity for belief is obviously due to the nature of the human brain – rocks can't form beliefs – there isn’t anything about the human brain which causes belief to always be certainty. Instead, the certainty of belief is simply a logically inherent, and therefore inevitable, feature of belief, for any form of intelligence. And so degrees of belief are actually logically impossible.

Theory versus experience

As I mentioned in chapter 1, although the idea that there are no degrees of belief can initially seem obviously contrary to experience, closer analysis reveals that it isn’t.

The theory states that, for example, our supposed belief with high but sub-certain confidence that there are six eggs in the fridge is actually certainty that it's highly likely that there are six eggs in the fridge. And it wouldn't be obvious from our experience of this mental state that our supposed belief wasn't actually the latter. Indeed, whether we believe claim X with a particular level of sub-certain confidence, or believe the claim X has sub-certain probability P with certainty, we’re equally uncertain about X, and so equally likely to question X, and so equally likely to question each belief.

Indeed, to be clear, the idea that belief is certainty doesn’t imply that we can't doubt any of our beliefs – which we obviously can – but that doing so constitutes the end of the belief – although, we may then at least believe that the content of that past belief is almost certain.

The theory also states that, contrary to our apparent experience, confidence and belief are different mental states. However, again, our confidence in X, and our belief regarding the subject of X, can, given the speediness of the brain, form within an unnoticeable fraction of a second of each other, and so these two events can seem to be part of the same event. And we can subsequently switch between these two mental states so quickly that they can continue to seem to be part of the same mental state. And it can therefore seem that our belief that X has sub-certain probability P is our sub-certain belief of X.

Also, the theory states that belief actually does involve, in itself, confidence in one sense: certainty in the sense of an absence of doubt.

Previous arguments against degrees of belief

I mentioned in chapter 1 that the idea that there are no degrees of belief actually isn’t new. However, none of the previous theoretical arguments conclude that our sub-certain confidence in claim X means that what we believe regarding the subject of X isn’t X, or that confidence and belief are separate mental states, or that degrees of belief are logically impossible, or that all belief is certainty in the sense of an absence of doubt. These arguments are presented by assistant professor of philosophy Andrew Moon in his 2017 paper 'Belief does not come in degrees'.

The first is based on an analysis of the grammar of our references to beliefs. However, I won’t go through the argument because its implicit premise is false. We can’t reasonably form conclusions about something based on how we refer to it. Even when we’re being honest, how we refer to something is simply a reflection of our current understanding of it, and therefore not necessarily a reflection of the reality of it. In order to reasonably form conclusions about something we must instead directly consider the thing itself. Moon also presents, and objects to, a grammatical argument in favour of degrees of belief, which fails for the same reason, although Moon of course objects to it for a different reason.

Before presenting the second argument against degrees of belief, Moon presents a counterargument to the argument that degrees of confidence in claims imply degrees of belief. He doesn't think that belief is simply confidence in a claim above a particular sub-certain level. If it were, then degrees of confidence would indeed imply degrees of belief. However, Moon also doesn't think that belief and confidence are separate mental states. He instead thinks that confidence is a property of belief. Therefore, he still agrees with the current understanding that belief involves, in itself, confidence. And he agrees that confidence comes in degrees. But he points-out that 'Although a property of something might come in degrees, the thing itself might not'. He gives the example of trees and their property of height: although tree height comes in degrees, trees themselves don't. That is, something is either a tree or it isn't. Therefore, he argues, degrees of confidence don't in themselves imply degrees of belief.

Note that although Moon thinks that there are no degrees of belief, he doesn't think that all belief is certainty, in the sense of either the maximum level of confidence or an absence of doubt. According to Moon, if our confidence in X is high but uncertain, then although this entails that we simply believe X, rather than have a high degree of believe of X, this belief involves that sub-certain level of confidence. Also, that sub-certain level of confidence implies that the belief also involves a level of doubt.

However, Moon doesn't actually provide a supporting argument for his premise that confidence is a property of beliefdespite criticising the idea that belief is simply a level of confidence for being a mere assumption. He simply states that references to particular beliefs involving particular levels of confidence are 'beyond reproach'. And the analysis in this chapter shows that Moon's premise is actually logically impossible.

The second argument against degrees of belief tries to show that it’s actually our intuition that there are no degrees of belief. However, again, I won’t go through the argument because its implicit premise is false. Whether or not the non-existence of degrees of belief is indeed our intuition, we can’t reasonably form conclusions about how some aspect of the world works, including our psychology, by uncritically following our intuition. Indeed, the usual immediate intuitive response to the idea that we don't have different degrees, or strengths, of belief is disbelief. The only way to reasonably form conclusions about how the world works is to instead perform a deliberate and careful analysis of the matter, although intuition can be a valuable source of ideas during that analysis.

The third argument against degrees of belief is a metaphysical argument. Metaphysics is the branch of philosophy that examines the fundamental nature of reality, and the argument involves an analysis of the concept of degrees of something. I'm not enough of a philosopher to be able to assess the logic of the whole argument. However, one of its steps depends on the idea that a small degree of confidence in a claim doesn't entail belief of it, and the argument doesn't attempt to justify this premise, even though some people think that any level of confidence in a claim above zero entails belief of it, however weak.

Also, the argument doesn't conclude against degrees of belief on the basis that they're logically impossible. Although the theoretical argument consists solely of conceptual analysis, it isn't a continuous chain of logical implication. The last part of the argument involves using an apparent condition for there being degrees of something to test the theory of degrees of belief. However, the argument only tests 'the two most plausible' versions of the theory that have been proposed, and so its conclusion that they fail the test doesn't constitute concluding that degrees of belief are logically impossible, given that it remains theoretically possible that one of the less plausible proposed versions could pass the test.

Also, aside from this argument's soundness, the fact that it reaches its conclusion on the basis of a metaphysical analysis of the concept of degrees of something means that it doesn't offer a psychological understanding of why there aren't degrees of belief – unlike the argument that I developed, which reaches its conclusion on the basis of a theoretical analysis of the psychology of belief and confidence.

The apparently flawed logic of these arguments partly explains why awareness of this theory today is low even among researchers in psychology and philosophy, and interest in it is even lower, and support for it is lower still. Regarding interest, there doesn’t seem to have been any experimental research into the theory.

The origins of the idea of degrees of belief

Again, the understanding that belief doesn’t require certainty, that we can have different degrees, or strengths, of belief, is widespread even within psychology and philosophy. For example, in the previously mentioned philosophical anthology 'Degrees of Belief' none of the twelve essays argues against this theory about the nature of belief. Its dominance will be due to the combination of the following five factors:

    1. Given that confidence in a claim is a feeling about whether it's true, and the mental state of belief is wrongly understood to concern the truth of the believed claim, we can wrongly assume that different levels of confidence in a claim, above a particular level, constitute different degrees of belief of the claim.

    2. Again, our confidence in X, and our belief regarding the subject of X, can, given the speediness of the brain, form within an unnoticeable fraction of a second of each other, and so these two events can seem to be part of the same event. And we can subsequently switch between these two mental states so quickly that they can continue to seem to be part of the same mental state. And it can therefore seem that our belief that X has sub-certain probability P is our sub-certain belief of X.

    3. Given that the claim It's highly likely that there are six eggs in the fridge is, like the claim There are six eggs in the fridge, positive about the idea of there being six eggs in the fridge, but is a weaker claim, we can assume that our belief of the former claim is a weaker belief than that of the latter. And this mistake is especially likely if we wrongly conflate the beliefs It’s certain that there are six eggs in the fridge and There are six eggs in the fridge.

    4. If we believe that the content of claim X is uncertain, but likely to some degree, then we don’t disbelieve X. And we can wrongly assume that if we don’t disbelieve X, and are positive about the probability of its content, then we must believe it to some degree – even though it’s actually possible to neither disbelieve or believe a claim.

    5. We can refer to our above belief as our sub-certain belief of X in order to avoid our non-belief of X being misinterpreted as disbelief.

And the falsity of the theory of degrees of belief hasn't been obvious given that, as I explained, it isn't obvious from our experience of supposedly believing X with sub-certain confidence that this mental state is actually our belief of the claim X has sub-certain probability P with certainty.

Why the certainty of belief matters

As I mentioned, whether we believe claim X with a particular level of sub-certain confidence, or believe the claim X has sub-certain probability P with certainty, we’re equally uncertain about X, and so equally likely to question X, and so equally likely to question each belief. However, this doesn't mean that the certainty of belief doesn't matter.

In the next chapter I show that, counterintuitively, the mere entrance of a claim into our mind, whether via reasoning, comprehension, imagination or recollection, causes us to believe it. If degrees of belief were possible then our belief of a claim upon it entering our mind could at least be an uncertain belief, and that uncertainty could motivate us to investigate the truth of our new belief. But the certainty of belief means that we always believe that claim without any uncertainty upon it entering our mind, which has profound implications for human psychology and therefore our lives.

Also, as I'll show in later chapters, the certainty of belief and our extreme credulity together explain cognitive biases, which are universal biases in human cognition. Since the early 1970s, researchers have discovered a large number of these biases. A Wikipedia page lists almost two hundred of them, and the list keeps growing.

I mentioned three of these biases in chapter 1: illusory superiority, confirmation bias and availability bias. Again, illusory superiority is our cognitive bias towards overestimating our qualities and abilities relative to those of other people. And confirmation bias is our cognitive bias towards interpreting, seeking, and remembering 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.

Confirmation bias and availability bias permeate all reasoning. However, illusory superiority is specific to a particular type of reasoningjudging our qualities and abilities – however frequent such reasoning may be. Another example of a cognitive bias that's specific to a particular type of reasoning is hindsight bias, which is our bias towards judging, with hindsight, that a past chain of events was objectively predictable.

Cognitive biases also account for our consistent logical, mathematical and statistical errors, and so together account for the whole of human irrationality.

There are multiple theories about why each cognitive bias exists, although there are only three types of such theories.

The first type of theory states that a particular cognitive bias is the product, or by-product, of a supposed cognitive heuristica cognitive rule-of-thumb shortcut that has become either hard-wired into our cognitive processes through evolution, or ingrained in those processes through learning. Cognitive heuristics supposedly develop to counteract our cognitive limitations, which are due to our finite amount of processing power, information and time.

The second type of theory states that a particular cognitive bias is simply a direct by-product of our cognitive limitations. And the third type of theory states that a particular cognitive bias is the result of particular emotions affecting our cognitive processes.

The cognitive heuristics explanation of cognitive biases has been dominant since the first biases were identified. Jonathan Baron, emeritus psychology professor at Pennsylvania University, subscribes to this theory in his popular 2008 textbook on rationality, Thinking and Deciding, which, according to the blurb, is 'the required text and important reference work for students and scholars of human cognition and rationality'.

Baron suggests that particular cognitive heuristics could be responsible for multiple cognitive biases, but also states ‘We have no good reason to expect a single account to apply to all [cognitive] biases that have been discovered’. However, although we indeed have no good reason to 'expect' such a unifying explanation, we equally have no good reason to expect there not to be one. Baron points-out that cognitive biases weren't discovered by searching for the effects of a single cause. But how we discover a natural phenomenon is irrelevant to its cause. Baron also points-out that cognitive biases could arise for many reasons. But they equally could arise for the same reason. And there are two good reasons to think that a unifying explanation of cognitive biases is a reasonable possibility which should therefore be explored.

First, all of the many different cognitive biases have one obvious but unique thing in common: they're cognitive biases.

Second, a central theme in the history of science is the development of theories that can simultaneously explain different phenomena that were assumed to require different explanations. For example, the 19th-century physicist James Clerk Maxwell showed that the apparently separate electrical and magnetic fields actually form a single electromagnetic field, and that many different forms of radiation – including light, radio, X-ray, microwave, infrared and ultraviolet – are simply different wavelengths of ripples in the electromagnetic field. And in the same century the biologist Charles Darwin showed that the myriad different species of plants and animals that exist today are all the product of the same process – evolution by natural selection – and are the current end points of the living branches of a single tree of life.

I'll show in later chapters that all cognitive biases are actually simply manifestations of the certainty of belief and our extreme credulity. And given that these two aspects of the psychology of belief are logically necessary, cognitive biases are also simply logically inevitable aspects of our psychology. Therefore the theories of the certainty of belief and our extreme credulity together provide a unifying explanation of all cognitive biases, and therefore of the whole of human irrationality.

As I mentioned in chapter 1, given that belief is the centre of human psychology, the theories of the certainty of belief and our extreme credulity will together revolutionise the field of psychology. And this will in turn have equally profound implications for every aspect of our lives.


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