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Rh of an animal or its parts: a dead body is not a man, nor a bronze hand a hand, nor the eye in a dead body really an eye. Rather, to describe an animal, one must show what it actually is in substance as well as form; and so with its several organs. He then argues that it is the soul or life which constitutes the essential nature of the animal. For "nature is spoken of in two senses, and the nature of a thing is either its matter or its essence; nature as essence including both the motor cause and the final cause. Now it is in the latter of these two senses that either the whole soul or some part of it constitutes the nature of an animal."

Nature always seeks an end,—a famous Aristotelian statement; and the end is the final cause, which in the case of animals is the soul or the life of the animal, the full functioning of its nature. Logically, that is, in thought, this final cause or end is prior to the motor cause; "For this is the Reason, and the Reason forms the starting point, alike in the works of art and in works of nature." With a builder the final cause is the construction of a house; in nature it is the making of an animal. "In the works of nature the good end and the final