Piaget's treatment of the actual development of human cognition from birth to adulthood is one of the best-known aspects of his thought, and is embodied in the theory of stages through which the individual necessarily passes over this period of his life. The sources of the stage theory are quite distinct from those of the structural theory of cognition. Actually, Piaget's lifelong search for a science of epistemology has occurred 'reciprocally'. The greatest pressure on him originally seems to have been the search for a union between experimental psychology and philosophy, and even his earliest views show a constant interaction between conclusions drawn from what are claimed as the experimental situation and a preconceived logicomathematical model (see Boring, ]969). It is from the former that the stage theory is principally derived whereas the structural theory is largely attributable to the latter. Piaget's working life seems to have alternated somewhat between these two positions. Strongly influenced as he was by theoretical considerations when young, his first five principal publications relate to work carried out with children. The period from the later 1930s through to the early 1960s seems to have been one of extensive theoretical elaboration, although experiments continued to be carried out in large numbers. More recently, experimental activity seems to have become increasingly prominent again as the overall trend of Piaget's work and writing in later life has been to concentrate increasingly upon the deeper implications of his experimental work. This is borne out by the relatively recent publications Success and Understanding (1975), The Grasp of Consciousness (1977), and particularly The Development of Thought (1978).