Humans: software snags

Are human beings rational? What sort of question is this and how should we answer it? This book takes its title from its concluding sentence: "Anyone who has confidently asserted either that humans are rational or that humans are irrational does so on the basis of incomplete empirical evidence and unsupported conceptual claims; in other words, she has taken a strong stand on the question of human rationality without good reason."

This seems to run counter to what we would normally expect. Asked whether human beings are rational, sensible people will reply that sometimes they are and sometimes they are not. Without doubt, sound reasoning plays a part in our lives, for if we had no regard at all for proper principles of logic, probability and evidence, our beliefs would end up no better than random guesses at the truth, and we would fail to meet up with food, shelter, friends, or any of the necessities of life. But obviously, tiredness, haste, emotion and the like can cloud our judgement and induce us to believe things against all evidence and logic. But I think that most people would say, before starting to philosophise, that the underlying tendency of humanity is to be rational and that it is only when some local disturbance upsets this general influence of reason that we fall into irrational belief. This is precisely what David Hume says in the Treatise of Human Nature of 1739: "Our reason must be considered as a kind of cause, of which truth is the natural effect; but such a one as by the irruption of other causes, and by the inconstancy of our mental powers, may frequently be prevented." Hume would classify this fact about human beings as one which requires experience to establish: it is not a truth which can be worked out in abstract just by reflecting on the implications of the pure concepts of logic, probability, evidence and truth. It is a fact about human beings which forms part of the science of human nature, based on observation.

Edward Stein largely agrees with this commonsensical view. He devotes much of his book to criticising alternatives. Philosophers often attribute rationality too quickly: some argue that sense can be made of beliefs only on the assumption that they generally tend to be true ("charity"); others that rationality is constituted by a balance between actual human practice and abstract principles of inference ("reflective equilibrium"); yet others that humanity must be rational to have evolved at all, overlooking that the truth of beliefs may in some contexts be systematically lacking in survival value.

Stein's criticisms of these rather unworthy positions are professional, careful and cogent. His positive proposal against Hume's common sense is less conclusive. Much hangs on the distinction between being rationally programmed but with odd disturbances to the smooth running of the programme ("performance errors") and not being rationally programmed in the first place. Stein may risk being too ready to interpret psychological evidence of systematic irrationality to mean that people are innately wired for irrationality rather than being innately prone to diverge from rational norms in diverse specifiable contexts. The hope for Hume is that the simpler explanation will always involve the background of general rationality because the norms of right reason are simpler than the patterns of systematic error. The plain view ("the standard picture") is still alive, and the nonphilosopher need not yet yield. This book's very considerable value is for professionals, on account of both style and content.

J. D. Kenyon is fellow and tutor in philosophy, St Peter's College, Oxford.