Still, people have a lot of obvious and important things that other species do not–speech, rationality, culture and the rest. Comparison must deal with these. I have tried to discuss some of the most important of them, not attempting at all to deny their uniqueness, but merely to grasp how they occur in what is, after a primate species, not a brand of machine or a type of disembodied spirit. I have tried to show these capabilites as continuous with our animal nature, connected with our basic structure of motives.

Introduction, Beast and Man: The Roots of Human Nature (1979).