Skinner was the leading behaviorist in psychology; he built on the work of John B. Watson, and added the idea of operant conditioning. These two American psychologists paid no attention to mental states and 'thinking' (terms they thought were unscientific), but dealt only with visible behaviors. Skinner's work had effects on education (programmed learning) and on behavior therapy for various psychological problems. He was the Edgar Pierce Professor of Psychology at Harvard University from 1958 until his retirement in 1974, and then Emeritus Professor until 1990.