ASSIMILATION, a term used in psychology to denote the mechanism by which two or more mental processes are drawn into a single unitary whole. What takes place in psychological assim ilation can be understood by considering the way in which lines slightly different in length are taken to be equal, or musical inter vals slightly out of tune (as in equal-tempered musical instru ments) assume the character of the pure intervals. In more com plex mental formations, such as perceptions, recognitions, memo ries and emotions, the parts lose their individual independence and form a single unit of experience.