B. F. Skinner (1904-1990), saw both humans and animals as empty organisms behaving like machines. He introduced instrumental (operant) learning and the concepts of reinforcement and punishment. Reinforcement refers to the conditions where the behavior occurs again because the obtained stimulus is positive (e.g., good treatment). Punishment refers to the conditions where the behavior is not likely to occur again because the obtained stimulus is negative (e.g., bad treatment).