The striking analogies between the ideas of Freud and Friedrich Nietzsche, whose works were published from one to three decades before those of Freud, have been commented upon, but no previous systematic correlation of the ideas of Nietzsche and Freud has been made.

The major works of Nietzsche were read, and each possible analogy to an idea later broached by Freud was correlated by a systematic review of his works. Any references to Nietzsche in Freud's writings and reported conversation were culled.

Concepts of Nietzsche which are similar to those of Freud include (a) the concept of the unconscious mind; (b) the idea that repression pushes unacceptable feelings and thoughts into the unconscious and thus makes the individual emotionally more comfortable and effective; (c) the conception that repressed emotions and instinctual drives later are expressed in disguised ways (for example, hostile feelings and ideas may be expressed as altruistic sentiments and acts); (d) the concept of dreams as complex, symbolic “illusions of illusions” and dreaming itself as a cathartic process which has healthy properties; and (e) the suggestion that the projection of hostile, unconscious feelings onto others, who are then perceived as persecutors of the individual, is the basis of paranoid thinking. Some of Freud's basic terms are identical to those used by Nietzsche.

Freud repeatedly stated that he had never read Nietzsche. Evidence contradicting this are his references to Nietzsche and his quotations and paraphrases of him, in casual conversation and his now published personal correspondence, as well as in his early and later writings.