It is a commonplace that philosophers, psychologists and economists often talk about rational belief or rational behavior, but they seem to mean different things by it. Is this because

(a) there is no single thing called rationality, it is fundamentally an ambiguous term;

(b) rationality is vague, there are no specific criteria for it, but we can recognise paradigm cases of rationality and irrationality;

(c) rationality, like beauty, is in the eye of the beholder, too much depends on the perspective of the agent to define it objectively;

(d) rationality might be definable, but current studies are disconnected, and nobody has succeeded in giving a unified account.

  • I definitely see how this is a big question in philosophy. What I don't see is how we can answer it in an SE format. Can you give some clearer parameters of what a good answer would like to this question?
    – virmaior
    Commented Nov 12, 2015 at 1:59
  • I'd be happy to see a rough sketch of what a unified account of rationality might be like, or alternatively an explanation of why it is impossible.
    – Bumble
    Commented Nov 12, 2015 at 3:06
  • What it might look like is shown in Kant's three Critiques, and the breakdown of his account is what gave us the analytic/continental divide. Kant limited reason to clear room for faith, and found a place for rational religious and aesthetic expression, but under the auspices of scientific rationality as its privileged form. This was retained by neo-Kantians, even Cassirer, but nothing of the sort would be acceptable to a continental philosopher. Psychologists and economists may have some vague consensus on what is rational behavior, but there is no hope for philosophical rationality.
    – Conifold
    Commented Nov 12, 2015 at 22:15
  • Since most people's underlying logic is the complete classical logic, it's possible to have a unified account of rationality at least in deductive part in theory. Actually when you see different people acting differently to a same problem/question, they're already perhaps all rational in form according to their own hypothesis. Since there're no two same leafs in the world everyone's hypothesis/axioms is always different, thus you'll never see uniform concrete actions... Commented Aug 13, 2022 at 3:06

2 Answers 2


Since this is multiple choice, I'll choose (b) and (c), with misgivings.

It is not hard to locate paradigms of what is meant by "rationality." At the etymological and stripped-down, Hobbesian level, rationality is a coherent, reliable system of input-output calculations. The model for Hobbes and many others was Euclid's axiomized geometry. It is "in the eye of the beholder" in the sense that the correlating axioms, assumptions, applications, and social contexts of such systems will vary.

The problems in arriving at one, "unified account of rationality" are numerous, famous, and fraught, constituting a virtual history of philosophy.

First, there is the problem of the axioms, as the case of Euclid's fifth postulate famously illustrates. Axioms must ultimately be grounded in consensus and intuitions, which are unreliable leaps of faith, historically variable, and always susceptible to skeptical demolition, as Berkeley, Hume, and others demonstrated. And one can always reason from "biblical axioms" or "racialist axioms" and be perfectly rational, in the eyes of some beholders.

Second, the reliable symbolic systems of mathematics and logic must be translated into value-laden, oral languages, where all sorts of "bewitchments" await. This exorcising task inspired the logical atomism of Frege, Russell, Wittgenstein, and others, all of whom conceded defeat, some of whom also conceded that the "irrational," evolving plasticity of language was perhaps just as well.

Third, any rational system that wants to advance beyond tautology must attach itself to the slippery contingencies of experience. Carnap and others attempted to stick manipulable symbols to sense data. Kant demonstrated how the analytical concepts of reason could produce synthetic conclusions about experience, but only at the sad cost of truncating reason and possible knowledge.

Fourth, all rational systems that extend their domains must live in doubt of their own internal coherence. Such doubts were put to rest by Godel's mapping of Russell and Whitehead's great attempt at a "universal account of rationality." No rational system, even cleansed to consistent symbols, could ever "prove itself" or ensure it own completion and consistency. Game over.

So, even apart from vague, colloquial uses of the term "rationality," we are a left with a historical scrap heap of "expert-built" rational systems, which remain perfectly useful, but are unlikely to be unified into a single account to everyone's satisfaction.

But people never give up. Set theory is said by some to be the most fundamental, unified account of various rational systems, though I wouldn't know. Efforts are underway to employ a Quinean naturalized epistemology linking set theory with perception-based "cell assemblies" theoretically accessible to neural-cognitive tracking. Game reboot.


Reason (rationality) is one of the archetypal philosophical problems. When sciences adopt a term so overloaded, it is not by accident. Scientific hypotheses are usually formulated over a dogmatic background. When scientists borrow a philosophical term, they are actually taking a stand, justified by desirable ethical/practical implications, and this is especially true in the humanities. In other words, they are choosing a specific philosophical approach (materialism, empiricism, idealism,...), and pragmatically converting it into doctrine. There's nothing wrong with that, it's part of the game. Science needs to move fast, and travel light.

Philosophers may have quarrels and narcissistic tendencies, but they usually do not expect to reach some form of perfect agreement about the definition of concepts. "Unification" is not exactly a philosophical project, which is why I wouldn't hold my breath waiting for it to happen any time soon.

Your concern may come from the fact that many scientific doctrines that are based on some notion of individual rational behavior happen to be in critical condition nowadays. Not just psychology and economics, but also sociology, political science and law studies, among others. It is probably a good moment for the practitioners in these fields to return to philosophy for inspiration - and I believe this is happening.

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