The Soul consists of two parts, the Irrational and the Rational (as to whether these are actually divided, as are the parts of the body, and everything that is capable of division; or are only metaphysically speaking two, being by nature inseparable, as are convex and concave circumferences, matters not in respect of our present purpose). And of the Irrational, the one part seems common to other objects, and in fact vegetative; I mean the cause of nourishment and growth (for such a faculty of the Soul one would assume to exist in all things that receive nourishment, even in embryos, and this the same as in the perfect creatures; for this is more likely than that it should be a different one).

– Aristotle, Nicomachean Ethics