Sigmund Freud (1856-1939), Austrian psychologist. Freud theorized that mental illness could have psychological as well as physiological causes. He believed that the mind contains conscious and unconscious levels. bad memories are repressed and stored unconsciously, but may still affect behavior. Freud studied the unconscious by analyzing subjects' dreams. These studies led him to believe that sexual impulses were responsible for neuroses. Freud is considered one of the great creative thinkers of modern times although his work remains controversial.