Similarity and Difference in the Conceptual Systems of Primates : The Unobservability Hypothesis

The Two Faces of Diversity Biologists readily talk of genetic, physiological, morphological, behavioral, and organismal diversity. Indeed, every taxon is ultimately defined on the basis of some combination of variation at each of these levels of description, most of which are readily apparent and uncontested. Mammals are endothermic, whereas reptiles are exothermic. Birds can fly. Fish breathe underwater. Octopi emit ink jets; whales and dolphins emit sonar signals. Curiously, however, whereas biology was more or less founded on the tenets of diversity, psychology has exhibited a strong reluctance to embrace the idea of diversity among species – particularly among groups of closely related species, such as humans and other primates – focusing instead on trying to uncover universal laws of learning (for different perspectives on this problem, psychological diversity-qualitative differences in the mental systems of different species-has 2 not prominently entered the lexicon of comparative psychology, let alone become part of the general notion of biological diversity. Ironically, we believe at least part of this resistance stems from the operation of uniquely human systems that cause us to form strong empathic and intellectual identifications with other species (particularly with other primates). From the opposite standpoint, however, humans have always possessed an unwavering interest in what makes us psychologically distinct from other species. Indeed, there is no shortage of time-honored proposals on this point. Just to list a few: humans alone have evolved natural language (a communicative system that involves deep underlying semantic and syntactical structures); humans alone construct complex inventions and alter our environment in profound ways; humans alone have a meaningful culture; and the list goes on. Surely, then, when focused on this half of the question, it is easy to embrace the idea that we are very different from even our closest living relatives. In the abstract, of course, this tension between similarity and difference does not present a real barrier to thinking about cognition from an evolutionary perspective (for recent attempts to explore cognitive evolution in a diverse array of taxa, about: similarity and difference. The similarities among taxa at each nested level of the biological hierarchy are precisely what force us to the conclusion that life on this planet ultimately constitutes a single genealogy; traced back far enough, even elephants and fruit-flies have a common ancestor. Conversely, the differences among taxa provide the evidence that this unbroken genealogy of life is, and has been, sculpted by …