A psychic process by which a new conscious content is articulated with similar, already existing contents in such a way that it is understood. (Compare assimilation.)
Sense-perceptions tell us that something is. But they do not tell us what it is. This is told us not by the process of perception but by the process of apperception, and this has a highly complex structure. Not that sense-perception is anything simple; only, its complex nature is not so much psychic as physiological. The complexity of apperception, on the other hand, is psychic. [“The Structure of the Psyche,” CW 8, par. 288.]
Jung distinguishes active from passive apperception. In active apperception, the ego grabs hold of something new and comes to grips with it. In passive apperception, the new content forces itself upon consciousness, either from outside (through the senses) or from within (the unconscious). Apperception may also be either directed or undirected.
The process of integrating outer objects (persons, things, ideas, values) and unconscious contents into consciousness.
Assimilation is the approximation of a new content of consciousness to already constellated subjective material. … Fundamentally, [it] is a process of apperception, but is distinguished from apperception by this element of approximation to the subjective material. … I use the term assimilation … as the approximation of object to subject in general, and with it I contrast dissimilation, as the approximation of subject to object, and a consequent alienation of the subject from himself in favour of the object, whether it be an external object or a “psychological” object, for instance an idea. [“Definitions,” CW 6, pars. 685f.]
© from Daryl Sharp’s Jung Lexicon, reproduced with kind permission of the author.