Through the development of a novel observational method, Sigmund Freud made possible the collection of reliable data about man's inner life. The scientific hypotheses he formulated about these formed the initial version of psychoanalysis. Many of these first thoughts have had to be revised in the light of subsequent scientific findings about the operations of the central nervous system, but even these refuted propositions often had much heuristic value. Despite the passage of a whole century, many Freudian hypotheses have retained their scientific standing. Most important among these was Freud's realization that human thought is usually unconscious. His understanding of the role of the automatic repetition of basic patterns of behavior, of the fateful consequences of early childhood emotional vicissitudes in structuring enduring mental dispositions, and of the distinction between two distinct modes of thinking are the most significant among his many contributions.