John B. Watson (1878–1958) is considered the founder of Behaviourism, having formally established it with his 1913 paper 'Psychology as the Behaviourist Views It.' B.F. Skinner later extended behaviourism through Operant Conditioning. Watson argued that psychology should only study observable behaviour, not inner mental states. Sigmund Freud is associated with psychoanalysis; Piaget with cognitive development. Watson founded behaviourism; Skinner extended it.