Many philosophers of the Western tradition have considered the relationship between human beings and other species of animals. Although some have been impressed with the obvious similarities in organic structure and behavior, most have tried to draw a clear distinction between the two. Only recently have a few taken seriously the extent of our moral obligations to fellow sentient beings. Plato, Aristotle, Augustine, and Aquinas all supposed that the ability to reason makes human souls uniquely superior to those of all other beings. Descartes regarded it as a consequence of mind-body dualism that non-human animals are mere machines incapable of thought of any sort. But Locke, £tienne de Condillac, and Bayle noticed that many of the capacities and activities exhibited by animals are similar to those of human beings, and La Mettrie argued that purely mechanistic explanations could be given for both human and animal behavior.