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.