Although poets and mystics had revealed a world of unconscious and irrational behavior, many scientifically oriented intellectuals under the impact of Enlightenment thought continued to believe that human beings responded to conscious motives in a rational fashion. But at the end of the nineteenth century, Viennese doctor Sigmund Freud (1856 –1939) put forth a series of theories that undermined optimism about the rational nature of the human mind. Freud’s thought, like the new physics, added to the uncertainties of the age. His major ideas were published in 1900 in The Interpretation of Dreams, which laid the basic foundation for what came to be known as psychoanalysis. According to Freud, human behavior is strongly determined by the unconscious—former experiences and inner drives of which people are largely oblivious. To explore the contents of the unconscious, Freud relied not only on hypnosis but also on dreams, which were dressed in an elaborate code that needed to be deciphered if the contents were to be properly understood. Why do some experiences whose influence persists in controlling an individual’s life remain unconscious? According to Freud, repression is a process by which unsettling experiences are blotted from conscious awareness but still continue to influence behavior because they have become part of the unconscious. To explain how repression works, Freud elaborated an intricate theory of the inner life of human beings. Although Freud’s theory has had numerous critics, his insistence that a human being’s inner life is a battleground of contending forces undermined the prevailing belief in the power of reason and opened a new era of psychoanalysis, by which a psychotherapist seeks to assist a patient in probing deeply into memory to retrace the chain of repression back to its childhood origins, thus bringing about a resolution of the inner psychic conflict. Belief in the primacy of rational thought over the emotions would never be the same.