Human concern with the moral status of non-human animals can be seen to stretch quite some way back into human history. In ancient Greece such concerns were considered to be very much a part of the ethical agenda, with thinkers on the issue being divided into four main schools of thought: animism; vitalism; mechanism; and anthropocentrism (Ryder 1989, chapter two). The leading light of the animist school was the renowned mathematician Pythagoras (circa 530 BC), who asserted the view that animals, like humans, were in possession of immaterial souls which, upon death, would be reincarnated in another human or animal body. In accordance with his beliefs, Pythagoras practiced kindness to animals and adhered to a vegetarian diet. Vitalism, of which perhaps the most famous exponent was Aristotle (384-322 BC), held to a belief in the interdependence of soul and body. Aristotle accepted the idea that human beings were animals, but he considered them to be at the apex of a chain of being in which the less rational existed only to serve the needs of the more rational. Mechanism held that both humans and animals were purely physical machines, and neither was in possession of the sort of soul that the animists and vitalists posited. Finally, anthropocentrism asserted that everything in the world has been created for the good of humans but, unlike the vitalism of Aristotle, rejected the idea of the essential ‘animality’ of humankind.