Sigmund Freud. Portrait of the Austrian psychologist Sigmund Freud (1856-1939). Freud established the idea that mental impairments could have pyschological as well as physiological causes. He theorised that the human mind contains conscious and unconscious levels; unpleasant memories were repressed and stored unconsciously, whilst still affecting behaviour. Freud studied the unconscious levels by analysing subjects' dreams. His studies led him to believe that sexual impulses were responsible for all irrational thoughts & behaviour (neuroses). Freud is generally acknowledged as one of the great creative thinkers of modern times.