Sigmund Freud (1856-1939), Austrian psychologist. Freud established the idea that mental disorders could have psychological as well as physiological causes. He theorised that the human mind contains both conscious and unconscious levels. Unpleasant memories are repressed and stored unconsciously, but they still affect the behaviour of a patient. Freud analysed dreams to study this unconscious level. His studies led him to believe that sexual impulses were responsible for irrational thoughts and behaviour (neuroses). This 1936 oil painting is by Austrian artist Wilhelm Victor Krausz (1878-1959).