INTROSPECTION. - It is the method of analyzing the appearance and content of psychic processes through self-observation, which is called i. when conducted systematically and according to precise scientific criteria. In this sense, a distinction is made between auto-i. and hetero-i.: in the former, the subject independently observes and describes the immediate contents of consciousness, whereas in the latter, they describe them by following the instructions of the experimenter. The foundation of i. lies in the capacity of spiritual consciousness to reflect upon its inner functions and to consider their objects and acts: «Intellectus supra seipsum reflectitur, secundum eamdem reflectionem intelligit et suum intelligere et speciem qua intelligit» (Sum. Theol., 1º, q. 85, a. 2). Although knowing is directed primarily toward the object, it simultaneously perceives its own act and the subjective structures of its functions or the species of the Scholastics: «Et sic species intellecta secundario est id quod intelligitur» (ibid.).
The method of i. was brought to prominence especially by the Denkpsychologie of the Würzburg school (Külpe) and had the merit of overcoming positivism and phenomenism in psychology. The method of i. was criticized in advance by Reid (Essay on the faculties of the human mind, I, 3; ed. Hamilton, t. II, Edinburgh 1863, p. 518 ff.) and especially by Kant, according to whom «inner sense - unlike outer sense, which can see objects in space in a state of rest and therefore observe them at ease - perceives the relations of its own determinations only in time and thus in their flow, where that constancy of observation is lacking which is nevertheless a necessary condition for experimenting» (E. Kant, Anthropologie, part 1º, l. I, § 4: Von dem Beobachten seiner selbst, in Werke, ed. E. Cassirer, VIII, Berlin 1922, p. 18). Similar criticisms of i. were advanced by Herbart, Comte, Höfling. A middle ground is that proposed by Gestaltpsychologie, according to which even the "ego" appears as a field of "structural organization," which makes it present in its contents and functions (cf. K. Koffka, Principles of Gestalt psychology, London 1936, p. 319), but by doing so one approaches the objective method of "behavior" (behaviorism). In philosophy, the method of i. received particular development in the intuition of Bergson and in the phenomenology of Husserl's school: however, this direction has leaned toward an i. in the idealist transcendental sense of consciousness.