De Anima, Book I c. 3:
The view we have just been examining, in company with most theories about the soul, involves the following absurdity: they all join the soul to a body, or place it in a body, without adding any specification of the reason of their union, or of the bodily conditions required for it. Yet such explanation can scarcely be omitted; for some community of nature is presupposed by the fact that the one acts and the other is acted upon, the one moves and the other is moved; interaction always implies a special nature in the two interagents.
In the contemporary account of the interaction problem, it is usually assumed that soul must itself be the physical or the organization of the physical. Aristotle, however, resolves the problem by making soul and body not two bodies, but an actual and potential life.