These are exciting times for philosophy and psychiatry. After drifting apart for most of this century, the two disciplines, if not yet fully reconciled, are suddenly at least on speaking terms. With hindsight we may wonder why they should have ignored each other for so long. As Anthony Quinton pointed out in a lecture to the Royal Institute of Philosophy a few years ago, it is remarkable that philosophers, in a sense the experts on rationality, should have had so little to say about the phenomena of irrationality (Quinton, 1985, ch. 2). There have been partial exceptions, of course. Descartes and Kant both touched on madness; and there were, notably, important philosophical influences on the development of modern psychiatry in the late nineteenth and early twentieth centuries (Zilboorg and Henry, 1941). Yet even John Locke, who was a doctor as well as philosopher, confined himself to a fair-l y superficial distinction between what we should now call mental illness and mental defect—those with, in Locke's view, respectively too many ideas and too few (Locke, 1960).