APPERCEPTION. - Term that came into use with Leibniz (v.), who, by distinguishing between perception and apperception, claimed to avoid the difficulties into which both the Cartesians and Locke fell: whereas perception is the inner state of the monad in representing external things, apperception is the consciousness or reflective knowledge of this inner state and belongs to the entelechies or more perfect souls (Principi della Natura e della Grazia, § 4, Italian trans., Bari 1912, p. 234).
In Kant, by contrast, apperception rises to the supreme function of understanding, as the transcendental "I think": it must accompany all representations as the a priori synthetic unity of the manifold that precedes and grounds all other forms of unification. Just as the synthesis of representations presupposes the a priori unity given by the categories (v.), so the synthesis of the categories presupposes that of the "I think" from which they derive (Critica della ragion pura, §§ 15-20).
In Herbart, apperception indicates the psychological dynamism of assimilating new material to past experience into an organized whole. In W. Wundt, apperception is both the de facto synthesis of experience and the principle of the synthesis operated by feelings (creative synthesis); and it is passive apperception when the unification is experienced as a "undergoing," active if it involves on the part of the subject processes of preparation such as attestation, expectation, completion and the like (Grundriss der Psychologie, 4th ed., Leipzig 1901, § 15, pp. 250, 260-61).