An individual has a theory of mind if he imputes mental states to himself and others. A system of inferences of this kind is properly viewed as a theory because such states are not directly observable, and the system can be used to make predictions about the behavior of others. As to the mental states the chimpanzee may infer, consider those inferred by our own species, for example, purpose or intention, as well as knowledge, belief, thinking, doubt, guessing, pretending, liking, and so forth. To determine whether or not the chimpanzee infers states of this kind, we showed an adult chimpanzee a series of videotaped scenes of a human actor struggling with a variety of problems. Some problems were simple, involving inaccessible food – bananas vertically or horizontally out of reach, behind a box, and so forth – as in the original Kohler problems; others were more complex, involving an actor unable to extricate himself from a locked cage, shivering because of a malfunctioning heater, or unable to play a phonograph because it was unplugged. With each videotape the chimpanzee was given several photographs, one a solution to the problem, such as a stick for the inaccessible bananas, a key for the locked up actor, a lit wick for the malfunctioning heater. The chimpanzee's consistent choice of the correct photographs can be understood by assuming that the animal recognized the videotape as representing a problem, understood the actor's purpose, and chose alternatives compatible with that purpose.

Ethologists and comparative psychologists have found that the social behavior, discrimination learning, and especially the communicative behavior of many animals are sufficiently versatile to call into question the customary denial that animals have mental experiences comparable to our own. Many of their behavior patterns suggest that animals have mental images of objects, events, or relationships remote from the immediate stimulus situation, as well as self-awareness and intentions concerning future actions. Our behavioristic Zeitgeist has inhibited investigation of such possibilities, but reopening these long-neglected questions requires no departure from the materialistic approach to biological and behavioral science. The basic question at issue involves a comparison of those aspects of human and nonhuman brain function that sometimes lead to what we call mental experiences, at least in our own species. It seems more feasible to devise objectively verifiable tests for the existence of these general categories of mental experiences than for that of many others. While many other kinds of evidence could well be used (with appropriate caution and all possible controls) to weigh the likelihood that animals sometimes know what they are doing, the experimental use of communicative behavior seems especially promising, because the latter could serve a function analogous to human language, which provides our best evidence about the mental experiences of our fellow men.

Two chimpanzees have demonstrated the ability to learn to use graphic symbols to ask one another for tools needed to obtain food. The chimps readily attended to and complied with one another's requests by selecting the appropriate tool from six distinct alternatives. They then shared the food obtained by this means and readily reversed the roles of tool requester and tool provider. Their joint accuracy was 92%, even when human experimenters were absent from the room. When they were prevented from using the graphic system (by deactivation of their keyboards), joint accuracy dropped to 10%, even though the animals could still see and hear one another and could gesture or vocalize freely. The acquisition of tool names revealed that words are at first closely and concretely linked to function, and hence that the traditional object-naming paradigm alone is a comparatively difficult starting point for the chimpanzee.

This work has demonstrated that two chimpanzees have been able to comprehend the symbolic and communicative function of the symbols they use. The results raise questions concerning chimpanzee research employing other kinds of symbolic communication systems. Apparently there is little evidence, apart from anecdote or uncontrolled tests, demonstrating that either Washoe or Sarah (chimps in other investigator's projects) has fully comprehended the symbolic nature and the communicative potential of their respective gestural and graphic token symbol systems.