The behaviour of animals and man may be approached from many points of view, but there is one feature of their behaviour which is outstanding in its importance and in its difficulty. I refer to the peculiar “adaptiveness” of animals' behaviour. The concept is vague, but everyone knows, roughly at least, what is meant by it. It is certainly one of the most remarkable features of animal behaviour. In psychiatry its importance is central, for it is precisely the loss of this “adaptiveness” which is the reason for certification.