"This integrative action in virtue of which the nervous system unifies from separate organs an animal possessing solidarity, an individual, is the problem before us."

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Plants, again, inasmuch as they are without locomotion, present no great variety in their heterogeneous pacts. For, when the functions are but few, few also are the organs required to effect them. ... Animals, however, that not only live but perceive, present a great multiformity of pacts, and this diversity is greater in some animals than in others, being most varied in those to whose share has fallen not mere life but life of high degree. Now such an animal is man.

— Aristotle