Reason, or rationality, is classically defined as deriving a conclusion from observations. Again, classically this is achieved by the application of logic. Aristotle explained it in this way. There are other means of reasoning involving intuition, imagination, or even faith. It seems that logic need not necessarily employed in rationality. Imagination is often employed in science. It might be helpful to explain what is meant by irrationality. Fundamentally, reason is the pursuit of truth. Irrationality appears to be an attempt to obscure the truth. Is an irrational reason a reason? So is any sincere attempt to reveal the truth a form of reason? Whether or not it employs logic? Even if it argues from a position of faith? So have the philosophers who have considered a definition of reason since Aristotle come up with a clearer definition?

  • what is reason?
    – user67675
    Commented Nov 14, 2023 at 7:58
  • 1
    That is the question. If something is defined it reveals what it is.
    – Meanach
    Commented Nov 14, 2023 at 8:01
  • 3
    See Reason and Rationality. See also SEP for linked entries. Commented Nov 14, 2023 at 8:15
  • 3
    There seems to be little point in providing answers to these questions because we've been seeing that even detailed, citation-based answers to them are not being reliably accepted. This creates an impression of debate-mongering with regards to these questions being posted. Is there any way for you to rectify this, Meanach? Commented Nov 14, 2023 at 13:36
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    Well then I am willing to vote to reopen this question (for the time being). I would encourage you to review the site FAQ/protocols regarding asking specified questions, for this should be of some benefit in understanding how to temper overgeneralization issues with respect to future questions. For better or worse, we are looking for details and particulars, here, and though philosophy is ambivalently general in many ways, there are ways to specify things appropriately. Commented Nov 14, 2023 at 14:42

7 Answers 7


Kant in his “Critique of Pure Reason” (CpR) discriminates between on one hand mind (= Verstand), and on the other hand reason (= Vernunft). Of course, both human capabilities to think rely on logic.

  1. The mind constructs concepts by applying four categories: Quantity of judgement, quality, relation, modality. See CpR B95. Mind uses these concepts to process the sense input, after it has been preprocessed by the two concepts of intuition space and time, see CpR B37ff. The output of the processing is named experience.

  2. Reason is the capability to form overarching ideas, named transcendental ideas. Different from the mind reason does not create concepts to construct experience, but creates rules to guide the activity of the mind:

“Thus we see that reason, in forming inferences, tries to reduce the great manifold of knowledge of the understanding to the smallest number of principles (universal conditions), and thereby to produce in it the highest unity.”(CpR 361)


By looking at the definitions of reason in Wikipedia and Britannica - as proposed by the initial comments - I find a contradiction (truth is missing in Britannica); to my understanding this contradiction poses a major philosophical problem.

Etymology and related words:

In the English language and other modern European languages, reason, and related words, represent words which have always been used to translate Latin and classical Greek terms in their philosophical sense. The original Greek term was "λόγος" logos, also the root of the modern English word "logic" ...

The word logos was a fundamental concept in Ancient Greek philosophers. For unknown reasons there exists a discrepancy between proposed definitions and the meaning of the word in the context it was used. Logos was generally considered as : a universal reason, immanent in nature, encompassing everything.

Since all Ancient Greek philosophers had logos as a fundamental concept, it is analyzed by Aristotle too. But he took this concept of logos and embedded it in his own metaphysical framework by making an interpretational shift. Logos was considered as an intrinsic property of the things themselves (beings) ("τι είναι" - "ον" - "what it is" - being/entity) as per se predication. link(7. Substance and Essence)...

By simplifying things:

In Aristotle (as well as other Ancient Greek philosophers) the reason of things (logos) is merged with its causes; Aristotle's causes are principles, foundations, the reason for being, or why something is the way it is;

In another person’s words:

It would be useful not to consider the ability of reason, which has similar equivalents in various Western languages, separately from the scope of “cause”, one of the essential meanings of “reason”. Reason means to know something with its cause, to understand it with its fundamentals. This is not only an etymological coincidence; it is significant as it is a manifestation of a tradition of thought that started in the Ancient Greece. According to the Ancient Greek, knowing everything is equal to knowing and understanding the same with its causes.

This was one of the most fundamental crossroads in Western thinking and philosophy. From then on, logos has been a concept of dispute and varying interpretations between philosophers, theology and others.

Without going into more details here, from what I understand, in the modern interpretation of reasoning this intrinsic property of "what it is" (logos) – i.e. to know something with its causes, to understand it with its fundamentals - is out casted and what is left, is a rational and logical only, way of seeking the truth;

I consider all this as a fundamental discrepancy in the way we perceive reality. Hopefully though, language has kept all these meanings - although obscured - in its usage:

  • What is your reason for missing school yesterday?


  • His desire to gain a promotion was the reason behind his underhand behavior.


  • While others panic, he shows reason and calm.


  • A good debater will reason, while a bad one might appeal to the emotions.

    (argue logically)

  • He wants to quit, but she is going to try to reason with him.

    (try to persuade)

  • He reasoned that there would be flooding, taking past rainfall into account.

    (support by facts)

  • From the evidence of a half-eaten sandwich, she reasoned that he must have left in a hurry.

    (infer, conclude)

  • Good point. Truth should be in the definition.
    – Meanach
    Commented Nov 15, 2023 at 8:06
  • A linguistic answer — a rare treat on this site! I will need to study and return to it. +1
    – Rushi
    Commented Nov 15, 2023 at 17:57

From personal experience, these are the concepts that fit the best with most documentation:

  • Knowledge is a model of the world. See some types of knowledge. Knowledge will always be incomplete and biased, because it is a model: models are abstractions of the terrain, not the terrain itself; abstraction essentially means simplification.
  • Thinking is the cognitive capacity of applying rules to facts of knowledge. It is closely related with cognition, which is mostly associated with the acquisition of knowledge.
  • Reason is the mechanism and the process of thinking. "Mechanism" refers to the static view of a system, the complement of the "process", the dynamic view of a system (e.g. you can describe a radio receiver as a static system, that is, the set of components, and as a dynamic system, that is, how do electronic components interact, how electricity flows).
    • Notice that reason has flaws (e.g. it is natural to make a wrongful deduction in multiple contexts, for example, under stress). That is normal: we all constantly make correct and wrong deductions, inductions, etc. Reason cannot be expressed formally: we can't draw UML dynamic or static diagrams of reason because we don't know enough about it. In other words, it is almost IMpossible to express reason formally (perhaps one day neuro-biology or AI will reach enough development to express it formally).
  • Logic is the formal expression of the rules of thinking. See the meaning of formal here. Many will say "no, you are confusing Logic with Formal Logic": there is no "informal logic". Logic is necessarily a formal expression. What is not formal is reason, as previously shown. The essential form of logic is propositional logic. I personally think propositional logic is the core axiom set, upon which all other axiom sets are based upon (e.g. the rules of chess are based on the rules of games theory, arithmetics, geometry, etc, which in ultimate term are based on propositional logic).

Must reason be founded in logic?

No. You can reason illogically (e.g. logical fallacies). Conversely, you cannot have an irrational logic (except if it is the result of illogical reasoning). It is logic which is based on reason, not the opposite.

  • +1 but your good answer shows the difficulties. May we "reason illogically"? But what is the meaning of "reason" here? A faculty? An activity? A set of rules? Commented Nov 14, 2023 at 11:54
  • @MauroALLEGRANZA 'But what is the meaning of "reason" here?': it is clearly defined in the text, it is a process ("I reason") and a mechanism ("using reason") 'May we "reason illogically"?', yes, reasoning (activating the process) illogically (not according to the rules of logic) occurs, for example, when you make a wrong addition calculation: you have reasoned, but you have missed or misapplied some rule of logic. I would say we can and it is normal, is what happens when you reason in a sleepy mood, under stress, or according to a logical fallacy.
    – RodolfoAP
    Commented Nov 15, 2023 at 6:44
  • Reason cannot be expressed formally: (snip) In other words, it is almost possible to express reason formally. This seems contradictory. Is this what you meant to say, or did you mean "almost impossible"?
    – Steve V.
    Commented Nov 15, 2023 at 7:42
  • Thanks for the observation and sorry, @SteveV., it is corrected now: it is almost impossible to represent -formally- the mechanism and the dynamics of reason.
    – RodolfoAP
    Commented Nov 15, 2023 at 12:26

Very few words can be precisely defined, since they are all open to shades of interpretation which in turn can be dependent on the context in which they are used, so the answer to your headline question is no.

Are there different categories of reason? Sure. The reason I had my breakfast was that I was hungry. Impact with the tip of the cue was the reason for the movement of the white ball. No logic involved there. Reasoning, in the most general sense I assume you meant, is the process of moving from one thought to another related one in order to reach a decision or develop your understanding of something. Logic, loosely speaking, refers to rules for certain types of reasoning, which, if followed, should ensure that one step in your chain of thought follows in a meaningful way from the preceding ones. But reasoning doesn't have to be explicitly logical, and faulty reasoning might not be at all logical.


If we identify reason as something which explains an event or action then reason can not be precisely defined especially in the human context. For example - A person may fight with his wife because he was feeling angry about his Boss at the office. It is an example of irrational reason.

A person can make a spontaneous choice to drink coffee or tea , without any apparent reason.(spontaneous reason driven by probability like a toss of coin)

In science , most phenomena have precise reason of cause and effect. Some processes are spontaneous (without any external cause) like radioactive decay.

There can be several categories of reason like rational reason or irrational reason, deep reason or shallow reason (as in chess) , strong reason or weak reason, reason based on digital logic or reason based on fuzzy logic(thumb rules), causal reason or spontaneous reason , reason based on knowledge or reason based on ignorance or delusion, real reason or fake reason etc..

  • Is an irrational reason a reason? Is it not an excuse? Example: The Nazis killed Jews because they hated them. At best, this is delusion. At worst, wilful evil.
    – Meanach
    Commented Nov 15, 2023 at 8:09
  • @Meanach Yes. Irrational reason can be a reason sometimes. The anger doesn’t die with the true cause. Anger can spread like a wildfire. Nazis killed Jews because of the difference in the faith. Deep down faith is in action. Commented Nov 15, 2023 at 10:18
  • An argument against faith? Nazism, a death cult, arose from faith? Thank you. you have reinforced my commitment to reason and disdain for faith. Long live atheism!
    – Meanach
    Commented Nov 16, 2023 at 15:31

Must reason be founded in logic?

You will have to make up your own mind about this.

Arguments are just the exhibit of reasons: Why do you bring an umbrella? Because the weather report says it is going to rain.

What is a reason? Clearly, it is just the protasis of a conditional: If it is going to rain, I better take an umbrella with me. The protasis explains the apodosis. It is the reason for doing what the apodosis says I am going to do.

So, the reasons we give are essentially language, not logic. Hence, we can give illogical reasons. Yet, we are clearly partial to logical reasons.

A reasoning is not just one reason, it is a chain of reasons understood as logically connected: It is going to rain, and I don't like being soaked, but an umbrella can protect me against being soaked, so, I better carry an umbrella.

The fact that arguments are not logic and that we can appreciate whether a reasoning is logical or not shows that logic is a normal innate capacity of (at least) human beings. Logic is valued. Irrespective of the facts of the matter, a reasoning perceived as illogical is dismissed out of hand: 2 + 2 = 4, so the earth is not flat.

Logic is valued because it is native. We value food because it is in our nature to sustain ourselves by eating food. Logic is valued because it works: If all Americans belonged to an Alien species, then Biden would be an Alien. We cannot fault the implication because logic is native to humans. We value it, because it works in our environment. It works because it is a native capacity, inherited through our DNA. If it didn't work, our ancestors wouldn't have survived, prospered and reproduced and we wouldn't even exist today. But we do, so, by Modus Tollens, logic works.

So, an argument needs not be logical, just as nobody needs to speak the truth. But, like truth, logic is valued, so, you better argue logically. And we are sometimes better at detecting illogical arguments than falsehood because logic is innate while truth isn't.

It is also better to reason logically because while logic works, the conclusion of an illogical reasoning has a 50/50 chance of being true. Not bad, and perhaps why illogical reasoning maybe is at all possible, but not very good either, and certainly not as good as logical reasoning. Logical people probably have better odds of survival in the long run.

Is reason itself founded on logic? Arguments are language and language is fundamentally logical, but we are obviously at liberty to argue fallaciously just as we are free to tell lies. Reason is probably 100% logic but arguments are what we do using language so we can tell what are other people's arguments when we hear them, but we cannot hear someone else's reasoning and so we canot tell whether other people reason logically or not, even when we hear their arguments, because people are at liberty to offer arguments which do not reflect their reasoning.

So, you will have to make up your own mind as to whether reason is fundamentally logical.


It is not immediately apparent what is meant by reason. So a robust definition is needed. Synthesising the answers here, it appears that logic is not necessarily employed. I accept the overarching idea raised by Kant in Critique of Pure Reason that reason is the ability to produce transcendental ideas. I think that it is possible that this concept can be further refined.

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