The different schools of psychology represent the major theories within psychology. Behaviorism became a dominant school of thought during the 1950s. Behaviorism suggests that all behavior can be explained by environmental causes rather than by internal forces. Behaviorism is focused on observable behavior. Theories of learning including classical conditioning and operant conditioning were the focus of a great deal of research. Psychoanalysis is a school of psychology founded by Sigmund Freud. This school of thought emphasizes the influence of the unconscious mind on behavior. Freud believed that the human mind was composed of three elements:the id , the ego and the superego. Humanistic psychology developed as a response to psychoanalysis and behaviorism. Humanistic psychology instead focused on individual free will, personal growth and self-actualization. Gestalt psychology is a school of psychology based upon the idea that we experience things as unified wholes. This approach to psychology began in Germany and Austria during the late 19th century in response to the molecular approach of structuralism. Instead of breaking down thoughts and behavior to their smallest elements, the gestalt psychologists believed that you must look at the whole of experience. According to the gestalt thinkers, the whole is greater than the sum of its parts. Cognitive psychology is the school of psychology that studies mental processes including how people think, perceive, remember and learn. As part of the larger field of cognitive science, this branch of psychology is related to other disciplines including neuroscience, philosophy and linguistics. One of the most influential theories from this school of thought was the stages of cognitive development theory proposed by Jean Piaget.