Sigmund Freud (1856–1939), Austrian psychiatrist. (Originally published 1917). A Difficulty in the Path of Psycho-Analysis, vol. 17, Complete Works, Standard Edition, eds. James Strachey and Anna Freud (1955).
This was Freud's conclusion in the light of the discoveries made by psychoanalysis that sexual instincts could not be wholly tamed, and that mental processes were unconscious and could "only reach the ego and come under its control through incomplete and untrustworthy perceptions."