drinking. In other words, the same response can be both a reinforcer and a punisher - at the same time and for the same individual. The equivalence of reinforcement and punishment is further suggested in this interesting fact: rats are either sensitive to both reinforcement and punishment, or insensitive to both; they are never sensitive to one but insensitive to the other. Premack introduced the concept of Theory of Mind, with Guy Woodruff, in an article published in 1978. This has proven to be a fruitful concept in psychology and neuroscience. For example, hundreds of articles have been published on theory of mind in fields ranging from comparative psychology studies of cognitive capacities of animals to human developmental psychology studies of infant cognition to social neuroscience studies of the brain substrates that mediate simulations of mental processes in other individuals. Premack's analysis of same/different led him and his associates to show that chimpanzees can do analogies. Sameness/difference is not a relation between objects (e.g., A same A, A different B) or properties, it is a relation between relations: For example: consider the relation between AA and BB, CD and EF on the one hand; and AA and CD on the other. AA and BB are both instances of same; the relation between them is "same." CD and EF are both instances different; the relation between them therefore is "same." AA is an instance of same, and CD an instance of different; the relation between them is "different." This analysis set the stage for teaching chimpanzees the word "same" for AA, and "different" for CD. When taught these words, chimpanzees spontaneously formed simple analogies between: physically similar relations (e.g., small circle is to large circle as small triangle is to large triangle), and functionally similar relations (e.g., key is to lock as can opener is to can). A nonverbal method for testing causal inference designed by Premack made it possible to show that both young children and chimpanzees are capable of causal inference. Premack demonstrated that chimpanzees can reassemble a disassembled face, placing the eyes, nose, mouth in the appropriate place. In addition he showed that chimpanzees are capable of symbolic behavior. After viewing themselves in a mirror wearing, on different occasions, a hat, glasses, necklace, and given the picture of a face, 48 hours later, the chimpanzees applied clay to the top of the head (hat), to the eyes (glasses),