With physics presenting an uncertain universe so unrelated to ordinary human experience, questions about the power and potential of the human mind assumed special significance. The findings and speculations of psychologist Sigmund Freud (1856–1939) were particularly disturbing.

Before Freud, most psychologists assumed that human behavior resulted from rational thinking by the conscious mind. By analyzing dreams and hysteria, Freud developed a very different view of the human psyche. Freud concluded that human behavior was governed by three parts of the self: the id, ego, and superego. The irrational unconscious, which he called the id, was driven by sexual, aggressive, and pleasure-seeking desires and was locked in constant battle with the mind’s two other parts: the rationalizing conscious — the ego — which mediates what a person can do, and ingrained moral values — the superego — which specify what a person should do. Thus for Freud human behavior was a product of a fragile compromise between instinctual drives and the controls of rational thinking and moral values.