Aristotle’s statement of the interaction problem

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.


  1. D.S. Thorne said,

    March 21, 2014 at 10:16 am

    I struggle with this. To say that mind is the actuality of the matter that is the body (that’s what Aristotle’s saying, right? Or do I have it wrong?) – to say this doesn’t answer everything. Aren’t we still left with the task of seeing, to the extent possible, how this cashes out in material and efficient causes, as modern scientist try to do? And again, why this specific configuration of matter (the human) has a potentiality for thought where a stone does not? This is something I’d like very much to understand!

  2. cd a said,

    March 21, 2014 at 9:03 pm

    Aristotle’s solution is open to mischaracterization by reason of incompleteness (perhaps necessary given the quotation). While the soul is to the body as act is to potential, the soul is not act simpliciter. That is, there are not two, but three degrees of act: operation (second act), first act / second potential, and first potential. Only operation (second act) is act simpliciter.

    Of body and soul, act simpliciter belongs only to the whole composite substance and not to one part apart from the other. Act simpliciter belongs only to the living body and not to the soul or body apart from one another. In other words, the operation (second act) of a living body is for that body to live.

    The soul is merely the principle of organization, or form, of the body such that it is that in the living body by which that body is capable (second potential) of life. In this way, the soul is the first act of the body. In other words, the soul is to body not as actually seeing is to the eye, but rather as the power of sight is to the eye. (II.1.412b25-413a5)

    Now, the body–considered not qua living, but merely qua matter–is capable of being a living body only in virtue of being capable of being the subject of a principle of organization that would make that body capable of life. In other words, body, qua matter, is twice removed from act simpliciter such that it has only the first potential for the life of that body.

    In short, Aristotle’s solution is to deny that there is any interaction to begin with by reason of their being no agents–there are simply two aspects of one and the same living body (namely, the body insofar as it is this type of body, i.e., as having this form, and the body insofar as it is matter, i.e., as capable of having this form). In other words, there is no interaction because there are no efficient causes at issue, only a formal cause and a material cause, for which unity cannot be a question. (412b6-9)

    Unfortunately, Aristotle’s account is often further obscured by concerns about the possible immortality of the human soul, which is a special case. In any case, it is not qua form of the body that the soul would be immortal, but rather qua having an operation not dependent on the body (i.e. qua intellect and will).

    cf. II.4.415b10-25. The soul is the source of movement not as an efficient cause, but inasmuch as the ability to grow, sense, etc. cannot be attributed to bodies in general–e.g. rocks are bodies but do not move of themselves–and so must be attributed to the type of body, whose proper cause is the form. The agent (efficient cause) is properly speaking either the whole living body (e.g. it is not the eyes that see, but the animal by means of its eyes) or what moves it (e.g. the object desired/needed, such as water and sunlight for most plants).

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