Thinking the Unconscious: Nineteenth-Century German Thought. Edited by Angus Nicholls and Martin Liebscher. Cambridge:  Cambridge University Press, 2010,  ix + 329 pp., $76.00.

Throughout his career, Freud reevaluated and defended the core psychoanalytic concept of the unconscious. In a paper from 1915, he insists “that our assumption of the unconscious is necessary and legitimate” (Freud 1915, p. 166). Necessary, because consciousness alone does not supply sufficient data to illuminate all cognitive and affective workings of the mind; legitimate, because we are justified in inferring that commonly accepted limits of fully knowing the external, physical world correspond to similar limitations of consciously knowing the internal, psychical world. Hence, the pursuit of psychoanalysis as a science of the mind's unconscious processes is as legitimate as Kant's critical exploration of the capacities of perception and reason: “Just as Kant warned us not to overlook the fact that our perceptions are subjectively conditioned and must not be regarded as identical with what is perceived though unknowable, so psycho-analysis warns us not to equate perceptions by means of consciousness with the unconscious mental processes which are their object” (p. 171).