After disposing of the Stoic materialism, according to which the soul, though distinct from the body, is, equally with it, an extended and resisting substance, our philosopher proceeds to discuss the theories which make it a property or function of the body. The Pythagorean notion of the soul as a harmony of the body is met by a reproduction of the well-known arguments used against it in Plato’s _Phaedo_. Then comes the Aristotelian doctrine that the soul is the entelechy—that is to say, the realised purpose and perfection—of the physical organism to which it belongs. This is an idea which Aristotle himself had failed to make very clear, and the inadequacy of which he had virtually acknowledged by ascribing a different origin to reason, although this is counted as one of the psychic faculties. Plotinus, at any rate, could not appreciate an explanation which, whatever else it implied, certainly involved a considerable departure from his own dualistic interpretation of the difference between spirit and matter. He could not enter into Aristotle’s view of the one as a lower and less concentrated form of the other. The same arguments which had already been employed against Stoicism are now turned against the Peripatetic psychology. The soul as a principle, not only of memory and desire, but even of nutrition, is declared to be independent of and separable from the body. And, finally, as a result of the whole controversy, its immortality is affirmed. But how far this immortality involves the belief in a prolongation of personal existence after death, is a point which still remains uncertain. We shall return to the question in dealing with the religious opinions of Plotinus.