On the Subject of the Soul

That the soul is a substance, is proved in the following manner. In the first place, because the definition given to the term substance suits it very well. And that definition is to the effect, that substance is that which, being ever identical, and ever one in point of numeration with itself, is yet capable of taking on contraries in succession. And that this soul, without passing the limit of its own proper nature, takes on contraries in succession, is, I fancy, clear to everybody.

For righteousness and unrighteousness, courage and cowardice, temperance and intemperance, are seen in it successively; and these are contraries. If, then, it is the property of a substance to be capable of taking on contraries in succession, and if the soul is shown to sustain the definition in these terms, it follows that the soul is a substance. And in the second place, because if the body is a substance, the soul must also be a substance. For it cannot be, that what only has life imparted should be a substance, and that what imparts the life should be no substance: unless one should assert that the non-existent is the cause of the existent; or unless, again, one were insane enough to allege that the dependent object is itself the cause of that very thing in which it has its being, and without which it could not subsist.