I believe that our understanding of formal logic is to formalize how a human brain is supposed to work. And this also makes sense when we think of mathematical problems that we only should get the correct answer if we follow the correct rules of inference. This also applies to simple human problems when you can model them in a logical way.

Based on that, how can so many people act so illogically? I am not talking about people who try to cheat or something like that but people who truly believe in things and fight for different types of causes which you can sometimes easily see that this causes hold a very clear logical hole. It's very common to see strong beliefs and disagreements because of the lack of logic.

Shouldn't this be rare excepetions? Is our logic missing some point?

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    artificial neural networks are an attempt to formally model the workings of the human brain (i.e. the way we think) and they are very different from logic. We are not logical because we are based on neural networks that generalize from experience rather than by logical deduction. Commented May 2, 2014 at 17:49
  • The issue can be seen as : laws of logic are more like laws of physics (i.e.natural laws : the planets cannot follow the "wrog" orbit) or like laws of ethics (i.e.human know what is good but often they "do bad things") ? Commented May 2, 2014 at 17:56
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    Neural netowrks are an attempt to model a linar/non-linear behavior with a topology based on the brain. But even with fuzzy logic, or a neural network, they all are based on mathematical models which are based (or at least are similar) to the classical logic. Maybe we could formalize the laws of nature by a complete different logic, but I believe one of the reasons that it is as it is, it's because it we think (or should) in a logical way. No? Commented May 2, 2014 at 20:28
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    Formalized logic is based on the most consistent human reasoning we can agree upon. Imagine the greatest minds working together to form a logical system. The layperson may have a hard time understanding it, but that does not make the system any less valid. The question then as to why people remain incoherent even after we've done great things is rather silly. If humans have invented spaceships and nuclear bombs and all manner of complex inventions, why are most of us so uninventive/not particularly brilliant? Because those things were invented by the best of us...
    – stoicfury
    Commented Jun 12, 2014 at 18:23
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    Our society is technological, yet the average person couldn't tell you how flipping a light switch turns on an electric light. When we talk about rationality, we're talking about the top thinkers and doers. The average person is a primitive child totally run by their lizard brain. It's a miracle that we managed to crawl out of caves.
    – user4894
    Commented Jun 12, 2014 at 19:03

15 Answers 15


Human reasoning evolved as a tool to ensure our survival on this planet. If a monkey miscalculated the distance of the next tree branch, he would be a dead monkey.

This means, that our brain has to create a somewhat consistent model of the world, yet by no means was there evolutionary pressure to think logically in a strict formal sense.

A lot of illogical fallacies are in fact encoded into our way thinking. Humans, for example, tend to think that if one object of a certain kind has a specific property, that all objects of those kind will have it.

That may be okay if we want to eat apples, and conclude that because one is not poisonous that all are not poisonous. Yet it may hinder us on social scale if we start to generalize population of other nations this way because of wrong perception of a few individuals.

This encoded way of thinking can be called our "common sense", which remains a useful tool, yet also a pretty dangerous one. Or as Albert Einstein said:

Common sense is nothing more than a deposit of prejudices laid down by the mind before you reach eighteen.

So I disagree that logic shows how our brains are supposed to work. Logic is our way to formalize valid reasoning as opposed to fallacious one in order to make clear where our human mind is prone to error.

Some aspects when people are disagreeing can indeed be rational vs irrational, as certain aspects of how the universe works has be shown quite well and has yet to be falsified (e.g. evolution as rational belief vs intelligent design as an irrational one).

Yet often people are disagreeing because they value things differently. It is hard to determine who is "irrational" if one wants to uphold freedom of speech while someone else wants to protect human dignity.

These groups of people are clashing not because one is irrational, but because they have different priorities in ethic values.

  • Quick 'common' sense vs slower reflection is expanded on in Kahneman's 'Thinking Fast And Slow'
    – CriglCragl
    Commented May 30, 2018 at 0:36

Another possibility that the question doesn't contemplate is that, potentially, people are "logical," in that from starting assumptions they follow logic to get conclusions, but they start with a different set of assumptions so come to seemingly very wrong conclusions.

For example:

1) People might spend money on "the wrong things," but a utilitarian might argue they are not being illogical, they have very different utility functions.

2) Given a set of evidence, they might come to the wrong conclusions, but a Bayesian would say that people had very strong priors before observing the new data, meaning their opinion changed little.

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    Another potential example: A reader of Euclid might logically deduce that that all triangles can be circumscribed, but without his parallel postulate, others might come to a different conclusion.
    – Lucas
    Commented May 2, 2014 at 18:57
  • In economics they also have an idealised rational agent, frequently could ntradicted by human behaviour en.m.wikipedia.org/wiki/Homo_economicus
    – CriglCragl
    Commented May 30, 2018 at 0:38

Logic is not based on human reasoning. It is a set of explanations about how to argue fruitfully. Arguments can be, and usually should be, recorded in some medium other than the human brain. And there are some ways of doing this that are better than others, so the relevant explanations are not limited to discussions of how the human brain works. Also, logic doesn't imply that people will actually follow those rules. The human brain is capable of universal computation so it can instantiate any set of rules, not just those that help to produce fruitful argument.

Also, it is not a good idea to think of mathematics in terms of getting the correct answer if only you follow the right rules of inference. Proofs should be thought of as tests of mathematical conjectures, not as ways of proving conjectures. See "Proofs and Refutations" by Imre Lakatos and "The Fabric of Reality" and "The Beginning of Infinity" by David Deutsch for better ways to think about maths.

  • I like your emphasis on nonbiological recording medium, but I am not Platonic enough to agree that logical is "not based on human reasoning." Commented Sep 10, 2015 at 15:02

Logic is different from reasoning. To open a lock I would need a key. To formalise this places it in the domain of logic. But people simply see that a key is required.

This means of course that one must know what a lock is, and what a key is for. But suppose we had someone who had never seen a lock before or key before. How would they react? Of course they may see that they have one object that has a hole in it and the other object is a stick like object. There is a general idea of putting a stick in a hole; but this is learnt behaviour too. So we judge what we are to do by reference with our past experience - this is called apperception in psychology (going back to at least Leibniz).

One can say that one persons world is different from anothers; and I mean that apperceptionally; to communicate these two worlds need to be in harmony; but in the real world which is in time to bring two worlds into harmony is a difficult task.

In this view, in essence, everyone reasons correctly (or the essence of the man - soul - reasons correctly); externally, which where we are; we see both imperfections/mistakes or perfections/correct.

  • "people simply see" i started (didn't finish) russell's analysis of matter. early on, he says that the physicist's first task is "aesthetic" - tho i can't remember quite what it was, perhaps cos i never studied physics at all.. anyway, i often wonder if aesthetics in general terms is a first step toward understanding, and how much of it is based on implicit memory
    – user6917
    Commented Sep 10, 2015 at 19:47
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    Arendt has a lot to say on this when thinking on Kants third critique - on judgement; judgement as taste ie aesthetics; it's not like Wildes 'Art for Arts sake' which is what I suppose people think of when they think on aesthetics. Commented Sep 10, 2015 at 19:59

Re: "how can so many people act so illogically?"

Human beings evolved in tribes, (by survival of the fittest tribes), so individual rationalisation is not elementary. Collective behaviour is the instinctive norm. Furthermore, the art of reasoning came late to the game. Trial and error is the default way of finding out what works, which is why ritual is so prevalent in our cultures.

For some suggested reading:-

Sigmund Freud - Group Psychology & The Analysis of the Ego

Sigmund Freud - Totem & Taboo

Elias Canetti - Crowds & Power

Konrad Lorenz - On Aggression

Pack behaviours are phylogenetic: "The motive power ... stems from instinctive behaviour mechanisms much older than reason and not directly accessible to rational self-observation." (p. 240)

And finally, C. Fred Alford - Group Psychology & Political Theory

"Political theorists should realize that the group is the state of nature, and that civil society is the product of the individual's struggle to separate from the group and develop a sense of self."


The assumption is flawed. Formal logic (as a branch of mathematics) is generally thought to exists independent of the existence of humans. Any non-human intelligence could have independently discovered the same mathematical structure (probably with a slightly different syntax, but corresponding). For instance, if A implies B and B implies C, then A implies C. That example logic is not tied to human reasoning, it is universal.

Now, if we consider logic as a branch of mathematics, we'd expect people to be no better at logic than at other branches of mathematics. They may be able to add 3 and 9 but 37 and 91 are harder, and few people can add 314 + 885 without paper. And numbers are particularly easy to manipulate.

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    I think you're presenting a minority view as the majority here.
    – Lucas
    Commented May 2, 2014 at 19:32
  • I agree that this common logic and also the mathematics can exist and be understood with or without the existance of humans. However, I believe that the logic has been formulated the way it is just because our brain tends to work that way. As I said above, maybe we could explain physics and laws of nature in a complete different way if our brain was different from what it is. Commented May 2, 2014 at 20:32
  • @FELIPE_RIBAS: There's no doubt about the existance of multiple formulations. Hamilton and Lagrange did come up with completely different formulations as Newton, yet all theories predict the same results. Aliens may have discovered any of these three formulations, or yet another. As long as the predictions made are consistent, it's just different words, but the same physics.
    – MSalters
    Commented May 2, 2014 at 22:55
  • What you talked about is causality not logic :) And it is true, humans even have problems with causality.
    – Asphir Dom
    Commented May 2, 2014 at 23:42

People have different priorities and different values, so, even from the standpoint of the formal logic, they're bound to act differently according to their values.

Otherwise put, what is illogical for you is not necessarily illogical for someone else.


Logic is not "in" individuals but "between" them. It evolved out of "arguing" with "reasons;" that is, out of the various forms of dialectic in Greek legal systems, oratory, rhetoric, and drama. It seeks the "irrefutable argument," using Euclid's axiomatic system as model.

What I believe is mistaken about your premise is the false dichotomy of "logic" and "illogical individuals." Logic evolves "between" individuals, who may instantiate it, to varying degrees, by way of a self-refining feedback loop.

Thus we can have many "illogical" individuals and a formalized, even governing logic, as in Hegel's "cunning of reason." This view, I believe, maintains the link with human reasoning, while avoiding a direct equation of the two... or the Platonic concept of an independent, bodiless, mathematical logic.


I do not think that this is a contradiction - it merely reflects that man are neither perfectly logical nor completely without logic. Logic is a tool which can be used to solved problems - but it is not part of our day to day toolkit do deal with problems. So people are using logic and sometimes ofter methods of thought to solve day to day problems and form beliefs. The rules are part of us - look at Dan Arielys work for an good introduction into irrational yet human behaviour.


So this is my first posting on the philosophers stack exchange, just keep that in mind if you have any critiques then let me know instead of just downvoting, this isn't reddit.

The way I've always seen it, is that logic, and mathematics, is a discipline. It's a thing you learn and get better at.

As others have mentioned, the brain is a complex machine. There is a lot that goes on under the surface, which would include a level of reasoning. This reasoning would be applied to your everyday situations. In fact infancy hits a stage where they utilize this reasoning on the most simplest of things, like turning things, and by things I mean EVERYTHING, upside down to see what happens. Who is to say that their experiments as a child were flawed, or happened to go horribly wrong for them, which is probably more along the side of phobias and the like.

There is a good handful, or wheel barrow full, of things that can mess with that reasoning. Another person mentioned tribes and how individual rationalism wasn't a thing that was accepted with open arms for the most part (seriously, count the great minds that were shunned, more on the side of not invited to dinner parties and less on the side of cast out of the village, by most of society in the past). Emotional reasoning is another thing that tends to get in the way of logic, which I imagine is at least one reason Socrates was so down on emotions and the senses.

So we have this basic human reasoning, which can be baffling at times. Formal logic is our counterpart to that. Yes, at some point in time somebody or some people created formal logic, or at least discovered it was a thing they could do. Which I have always thought of it as math but with statements. So you can then take your reasoning and thoughts and what have you, and put them into this mathematical formula and do all this weird stuff to prove them right or wrong. Most people don't do this. If they do this, and are still illogical, then that means they are using false statements from the get go, and sometimes there is no getting through to those types.

Edit: To be fair, most of those that use the false statements think they aren't false, its not because they are stupid, just misinformed for the majority of their life. It can also happen to anyone. For example, I believe a good number of you know about the Berenstein Bears, however most people did not know that the correct spelling is Berenstain Bears. Here is a fun article that claims we jumped into a parallel dimension. I mean, the example doesn't have the same scope as someone inherently believing black people are inferior, but I hope it gets the point across with it being fairly easy to learn something like that.

I'm sorry if I referenced too many other answers, I at least want to acknowledge that it was stated before to give credit to those who answered before me, as they went into greater detail on most those subjects than I did, and a good number of them dropped some pretty awesome links, or at least a title of a book.


Human beings evolved in tribes, survival of the individual and their offspring depends on their survival within the tribe, and on the survival of the tribe itself. Individual rationalization wasn't of benefit; hence there was no evolutionary pressure to think logically in a strict formal sense.

Human reason is strongly influenced by factors like emotion and instinct for survival. The art of reasoning came late to the game hence why emotional thinking has priority over logic.

Emotional thinking is a survival mechanism within a tribe or troupe. For example: if the Alpha male monkey attacks the C grade monkey, the best chance of survival for the other monkeys and their offspring is to side with the Alpha male monkey, regardless of whether it was in the right or not. Siding with the Dominant Force often means accepting a false narrative/ creating their own false reality, rejecting reality (denial) and rationalizing inconsistencies (primate rationalization/ cognitive dissonance).

Collective (tribal) behavior is the instinctive norm. "The motive power ... stems from instinctive behavior mechanisms much older than reason and not directly accessible to rational self-observation."

Speed of decision making would have been important for survival within a tribe. Emotional thinking allows for quick decision making by flipping the individuals desire one way or the other. Logic by contrast is a slow methodical problem solving tool, hence why it is not part of a Neurotypicals day to day toolkit for dealing with problems. By contrast, Psychopaths lacking the primative emotional system of Neurotypicals, nearly always think logically. Hence why Genetic Psychopathic traits are for the most part correlated with increased rates of success.


I believe that our understanding of formal logic is to formalize how a human brain is supposed to work. And this also makes sense when we think of mathematical problems that we only should get the correct answer if we follow the correct rules of inference. This also applies to simple human problems when you can model them in a logical way.

I think the point of strongest disagreement you may find is the claim that logic is to formalize how a human brain is supposed to work. That is not a universally held belief. As a key example, "love" is considered to be highly illogical, even by those who are in love at the time, and yet it is strongly considered by many that "love" is an essential part of being human.

I have found more success treating logic as a model which can represent many systems, and allow us to make predictions about those systems before they happen. It is useful in some situations where the situation permits useful predictions using logic, but not all situations support that. For example, in high stress situations, it is often a "gut instinct" that finds the small window of opportunity that could not be calculated using logic, and acts on it before the logic gets in the way.

If you are inclined towards logic and mathematics, I recommend looking into the work of Russel, Godel, and the other mathematicians who worked on the hairy edge of set theory, which entwines tightly with logic. They found situations where logic could not make inroads because much of modern logic is founded on set theory (in particular, First Order Logic, which is completely dependent on it). They eventually defined a new category of collection known as a "class," simply because there were things which could not be well defined within logic. If you hear obtuse phrases like "Could God create a rock so heavy that even he could not lift it?" the paradoxical nature of the question stems from individuals not understanding the difference between a set and a class -- the paradox becomes a non-issue once you understand the difference.

Also worth noting is that logic, if it reaches a "correct" answer, has the advantage of being easily proven (P vs NP fits into this issue). However, it does not adapt well to problems which are changing in nature while the logical analysis is being undertaken, and this is especially true in the presence of an "intelligent adversary" which can watch your logical analysis and respond to it. Many parts of the human condition are amicable to these rules: they explore things which change slowly enough or isolated enough to be picked apart by logic. However, other parts are not so friendly. Science provides one of my favorite experiments along these lines: the delayed eraser double slit experiment. This is a case with scientifically understandable results (it's repeatable, and not magic!), where most people's intuition fails utterly because they assume the particle under test is not "aware" of being observed. While assigning "awareness" to a particle may be overkill, it turns out to be a frustratingly good layman's metaphor for the particular entanglements that occur between the particle under test and your sensor.

When you get to the hairy edge of that which is not so friendly, you'll also likely find yourself next door to Godel's Incompleteness Theorems, which are a set of three theorems which identify interesting limitations on the capabilities of First Order Logic, along with his proof that Second Order Logic does not always admit proofs (proofs being very important for logic). I have had fun paraphrasing them into a short intuitive form which may be useful. The rule is that any sufficiently complex system (sufficiently complex being defined as being able to formally define the basic rules of arithmetic) the system must have at least one of the following characteristics:

  • Incomplete (does not provide a result for all values in its domain)
  • Inconsistent (provides values which lead to contradictions)
  • Unprovable (cannot ever be proven [using first order logic])
  • Intractable (can never be written down. In technical speak: the proof contains a non-recursively-enumerable set of statements)
  • Illogical (Simply doesn't follow the rules of logic... fair game)

(And for fairness, there's the work of Dan Willard, which works at a corner case of this where multiplication is not a total function, and he escapes these limitations for some narrowly defined systems)


I suppose you're talking about behaviour (there is v amusing cognitive research into syllogistic reasoning, which would make a better question IMHO) and perhaps belief.

As such, and because you explicitly rule out dishonesty, it's quite easily explained in terms of unconscious motivations, which they are not privy to.


Human reason is not composed entirely of logic. It is influenced by other factors. E.g. emotion and instinct. Most people desire to see the truth on thier own terms, instead of the truth on its own terms.


The simple, fundamental answer: "Logic" is fundamentally just mathematics. A + B = C. But our ideas as humans are so complex and so subjective, they don't fit into any math equation. Our brain has developed in such a way that can calculate the logic of these ideas, but it's not by any means a truly logical calculation, but more of a rough estimate.

Engineers working to develop Artificial Intelligence, I'm told, are having a big problem figuring out how to get their algorithms to work because they can't figure out how to teach a computer to make the "correct decision". They're trying to calculate "right and wrong" in binary and it wont work.

To solution will be to stop trying to calculate "right and wrong" because it doesnt exist. Only our personal emotional interpretations exist. As a society, and as cultures, we have had to unify our "personal interpretations" of right and wrong in order to live in harmony, but logical decision making will always be just a rough estimate, a very imperfect calculation because logical thinking is purely relative.

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