The progress of human knowledge depends on maintaining that touch of scepticism even about the most "unquestionable" truths. A century ago, Darwin's theory of evolution by natural selection was regarded as scientifically unshakeable; today, most biologists have their reservations about it. Fifty years ago, Freud's sexual theory of neurosis was accepted by most psychiatrists; today, it is widely recognized that his methods were highly questionable. At the turn of this century, a scientist who questioned Newton's theory of gravity would have been regarded as insane; twenty years later, it had been supplanted by Einstein's theory, although, significantly, few people actually understood it. It seems perfectly conceivable that our descendants of the twenty-second century will wonder how any of us could have been stupid enough to have been taken in by Darwin, Freud or Einstein.