Perhaps the most significant contribution Freud has made to modern thought is his conception of the unconscious. During the 19th century the dominant trend in Western thought was positivism, the claim that people could accumulate real knowledge about themselves and their world, and exercise rational control over both. Freud, however, suggested that these claims were in fact delusions; that we are not entirely aware of what we even think, and often act for reasons that have nothing to do with our conscious thoughts. The concept of the unconscious was groundbreaking in that he proposed that awareness existed in layers and there were thoughts occurring "below the surface." Dreams, called the "royal road to the unconscious", provided the best examples of our unconscious life, and in The Interpretation of Dreams Freud both developed the argument that the unconscious exists, and described a method for gaining access to it.
The Preconscious was described as a layer between conscious and unconscious thought—that which we could access with a little effort. (The term "subconscious" while popularly used, is not actually part of psychoanalytical terminology.) Although there are still many adherents to a purely positivist and rationalist view, most people, including many who reject other elements of Freud's work, accept the claim that part of the mind is unconscious, and that people often act for reasons of which they are not conscious. In a lecture at Clark University in 1910, he explains his new conception of the workings of the human mind and its rejection by fellow professionals and the public. "The arrogance of consciousness which for example rejects dreams so lightly, belongs quite generally, to the strongest protective apparatus which guards us against the breaking through of the unconscious complexes, and as a result it is hard to convince people of the reality of the unconscious, and to teach them anew what their conscious knowledge contradicts."
Crucial to the operation of the unconscious is "repression." According to Freud, people often experience thoughts and feelings that are so painful that people cannot bear them. Such thoughts and feelings—and associated memories—could not, Freud argued, be banished from the mind, but could be banished from consciousness. Thus they come to constitute the unconscious. Although Freud later attempted to find patterns of repression among his patients in order to derive a general model of the mind, he also observed that individual patients repress different things. Moreover, Freud observed that the process of repression is itself a non-conscious act (in other words, it did not occur through people willing away certain thoughts or feelings). Freud supposed that what people repressed was in part determined by their unconscious. In other words, the unconscious was for Freud both a cause and effect of repression...