This book, the culmination of forty years of theorizing about the moral status of animals, explicates and justifies society's moral obligation to animals in terms of the commonsense metaphysics and ethics of Aristotle's concept of telos. Rollin uses this concept to assert that humans have a responsibility to treat animals ethically. Aristotle used the concept, from the Greek word for "end" or "purpose," as the core explanatory concept for the world we live in. We understand what an animal is by what it does. This is the nature of an animal, and helps us understand our obligations to animals.