Originally published in 1983, the aim of this book was to discuss some fundamental problems of cognitive developmental psychology at the time. The theme which underlies the discussion is that scientific knowledge of the cognitive characteristics of other people starts from the cognitive instruments that we psychologist employ, viz. our theories, models, assumptions, methods of enquiry etc. Thus our scientific cognitive equipment not only provides the format in which cognition in other people is expressed, it also exemplifies, in some abstract sense, this cognition. The first part of the book deals with the concept of development in relation to the structure of developmental theories. It is argued that theories originate from (implicit) conceptual analyses of (implicit) final state definitions. Starting from this specific view on the nature of developmental theories, the second part of the book discusses perception and perceptual development.