With physics presenting an uncertain universe so unrelated to ordinary human experience, questions regarding the power and potential of the rational human mind assumed special significance. The findings and speculations of Sigmund Freud were particularly influential yet also deeply disturbing.

Most scientists assumed that the conscious mind processed sense experiences in a rational and logical way. Human behavior in turn was the result of rational calculation — of “thinking.” Beginning in the late 1880s, Freud developed a very different view of the human psyche. Basing his insights on the analysis of dreams and of hysteria, Freud concluded that human behavior was basically irrational, governed by the unconscious, a sort of mental reservoir that contained vital instinctual drives and powerful memories. Though the unconscious profoundly influenced people’s behavior, it was unknowable to the conscious mind, leaving people unaware of the source or meaning of their actions.

Freud described three structures of the self — the id, the ego, and the superego — that were basically at war with one another. The primitive, irrational id was entirely unconscious. The source of sexual, aggressive, and pleasure-seeking instincts, the id sought immediate fulfillment of all desires and was totally amoral. Keeping the id in check was the superego, the conscience or internalized voice of parental or social control. For Freud, the superego was also irrational. Overly strict and puritanical, it was constantly in conflict with the pleasure-seeking id. The third component was the ego, the rational self that was mostly conscious and worked to negotiate between the demands of the id and the superego.

For Freud, the healthy individual possessed a strong ego that effectively balanced the id and superego. Neurosis, or mental illness, resulted when the three structures were out of balance. Freud’s “talking cure” — in which neurotic patients lay back on a couch and shared their innermost thoughts with the psychoanalyst — was an attempt to resolve such unconscious tensions and restore the rational ego to its predominant role.

Yet Freud, like Nietzsche, believed that the mechanisms of rational thinking and traditional moral values could be too strong. In his book Civilization and Its Discontents (1930), Freud argued that civilization was possible only when individuals renounced their irrational instincts in order to live peaceably in groups. Such renunciation made communal life possible, but it left basic instincts unfulfilled and so led to widespread unhappiness. Freud gloomily concluded that Western civilization was itself inescapably neurotic.