Freud's Reasons for Believing in the Unconscious

(a) the post-hypnotic carrying out of suggestions held in the unconscious, 

(b) the evidences found through discovering the latent meaning of dreams,

(c) the discoverable bases for common slips or errors of speech, memory, action,

(d) the fact that ideas suddenly appear in the mind from somewhere outside conscious¬ness or even that problems are solved without awareness,

(e) the small amount in consciousness at any one time in comparison to the latent content of the mind,

(f) the fact that through psycho¬analytic technique various mental and physical symptoms are found to have their foundations in hidden mental life, and in general, the "analytic investigation reveals some of these latent processes as having characteristics and peculiarities that seem alien to us, or even incredible, and running directly counter to the well-known attributes of consciousness,"

(g) finally, "an incon¬trovertible proof" is found in that the assumption of the uncon¬scious helps us to construct a highly successful practical method by which we are enabled to exert a useful influence upon the course of conscious processes .

Source: Healy et al. 1930. The Structure and Meaning of Psychoanalysis  pp. 22, 24