David Premack (October 26, 1925 – June 11, 2015) was an American psychologist who was a professor of psychology at the University of Pennsylvania. He was educated at the University of Minnesota when logical positivism was in full bloom. The departments of Psychology and Philosophy were closely allied. Herbert Feigl, Wilfred Sellars, and Paul Meehl led the philosophy seminars, while Group Dynamics was led by Leon Festinger and Stanley Schachter. Research Premack started in primate research in 1954 at the Yerkes Primate Biology Laboratory at Orange Park outside Jacksonville, Florida. His first two chimpanzee subjects, Sarah and Gussie, started at the University of Missouri and traveled with him to the University of California, Santa Barbara, and then to the University of Pennsylvania, where he had nine chimpanzee subjects. Premack's first publication (1959) was a new theory of reinforcement (which became known as Premack's principle). It argued that the more probable response in any pair of responses could reinforce the less probable response—demonstrating that reinforcement is a relative, not an absolute property. This theory predicts six conditions, all of which have been supported by evidence: Reinforcement is a relative property. Responses A, B, C have a descending rank order of probability. A will therefore reinforce both B and C. C will reinforce neither. This suggests that reinforcement is an absolute property. However, B corrects this view. B will reinforce C, but not A. B is both a reinforcer and not a reinforcer. Reinforcement is therefore a relative property. Reinforcement is a reversible property. When drinking is more probable than running, drinking reinforces running. When the probabilities are reversed, running reinforces drinking. Historically, consummatory responses, eating and drinking, have served exclusively as reinforcers, but consummatory responses are, like any other response, subject to reinforcement. Reinforcement and punishment, traditionally contrasted as opposites, are in fact equivalent except for sign. If response A leads contingently to B, and B is more probable than A, A will increase in frequency (reinforcement); conversely, if A leads contingently to B, and B is less probable than A, A will decrease in frequency (punishment). The major contrast is not between reward and punishment; but between reward and punishment as contrasted with freedom. Freedom is the condition in which stimuli are freely (not contingently) available to an individual. When motorized running is more probable than lever pressing but less probable than drinking, then running reinforces lever pressing and punishes