It’s relatively easy to see how mental causation, both active and passive, would work on the interactionist model. We can simply apply our usual assumptions about per se efficient causal powers. What is more interesting is to work out how such causation could happen on the two emanationist models.
To keep things relatively simple, let’s start with non-rational animals, setting aside both the problem of intellectual acts and that of free will. We can safely ignore plants and other simple organisms, since they don’t seem to exhibit anything like mental causation. (If I’m wrong about this, it doesn’t really matter.) Let’s start with sensation. When an animal is appeared to redly, there seems to be a fundamental mental accident or operation in the picture. What is the bearer of this operation or accident? We are interested at this stage in what bears the accident or performs the operation most fundamentally. What, in the first instance, does the accident modify? For the pure emanationist, it can’t be the soul, so it must either be the whole organism or some proper part or parts. For the operationist, the operation of perception could be performed by the soul or by the whole substance. Which is it?
To my mind, the right answer is clearly, the whole organism (the substance). It is fundamentally the whole organism that senses and perceives. Of course, the proper parts of the organism are involved in an essential way in this accidental modification. When the organism sees redly, it eyes, its retina, its optic nerve, and parts of its brain bear accidents relating to the representation of redness. For the hylomorphist, these accidents are at least partly grounded in the sensory powers of the whole organism.
So, the soul does not sense or perceive. It enables the whole animal to sense and to perceive. The sensible object acts upon the organism, changing its internal state at a fundamentally holistic level. This seems to be Aristotle’s view (with Thomas’s agreement: In De Anima I, Lecture 10, par. 152):
Let’s turn then to the organism’s active power of responding behaviorally to stimuli, both internal and external. Here again, the action is fundamentally the action of the whole organism and only derivatively that of the nerves, muscles, sinews, and bones involved. The soul does not cause directly any change in anything—it is involved only indirectly, by causing (in a formal way) the active and passive powers of the whole organism and its material parts.
To say that the soul is angry is like saying it builds or weaves. For it is perhaps better to say, not that the soul is compassionate, or learns, or understands, but a man by his soul. These modifications occur by movements not so much in the soul as, in some cases, proceeding to it, and in others, proceeding from it, as sensation proceeds from things, whilst remembering proceeds from the soul to the motions or rests which occur in the sensitive organs. (De Anima I, 4 408b)
Many changes in the organism have entirely internal causes. Parts of the body act on the nervous system, and parts of the brain act on other parts. This poses no special problem, since hylomorphists can attribute both active and passive powers to material parts of the organism.
Animals can remember past events, and they can engage in internal acts of imagination and even planning. These are, like acts of overt behavior, most fundamentally activities of the whole organism. Can an organism act upon itself, producing changes on itself?
Aristotle seemed to reject the idea of a causal power that is essentially reflexive, i.e., a power of x to produce changes in itself but not in anything else (Physics VII.1, VIII.4, 5). Aristotle’s idea seems to be that to have the active power to produce change C in something, the power must in principle be exercisable in cases in which the patient and agent are not identical. Aristotle allows for cases of reflexive actions so long as the reflexivity is accidental or incidental: the case, for example, of a doctor who heals himself (Physics II.1). In this case, the doctor relates to himself as if he were a distinct person. The doctor perceives and manipulates his own body as if it were an external object in his environment. This is clearly not what is going on when an animal causes itself to remember or imagine.
Why was Aristotle so skeptical about the possibility of essentially reflexive causal powers? It may be that he was worried about the apparent lack of the possibility of non-proximity. Natural powers also manifest themselves when in the immediate vicinity of an appropriate patient. But if the agent and patient are identical, the patient is always proximate to the agent. What, then could explain why the power manifest when it does? In the case of the doctor healing himself, the patient becomes appropriately proximate when the agent perceives the relevant states or is enabled to treat the body in appropriate ways. The action is not essentially reflexive—it involves essentially external pre-conditions.
Aristotle’s reasons for dismissing essentially reflexive actions seem unconvincing to me. Why couldn’t there be chancy causal powers—powers that manifest some of the time when their preconditions are met but not always? And why couldn’t there be purely internal pre-conditions of the exercise of a reflexive power?
Some interpreters attribute Aristotle’s strictures to some sort of causal proportionality principle: for agent x to cause patient y to become F, x must already be F. If true, such a principle would rule out all reflexive action (including the self-healing doctor). In any case, I would argue that Aristotle applies this principle only with respect to the distinction between actuality and potentiality. That is, no merely potential agent can cause an actual change. And this seems reasonable but has no power to rule out reflexive action, except for the action of giving oneself actual existence.
In any case, when an organism engages in a prolonged activity of some mental sort, like the activity of imagining something edible, this activity (operation) is sustained by means of some parts of the brain acting on other parts, under the over-arching, emanationist supervision of the soul.
Aristotle’s arguments against self-movers in Books VII and VIII of the Physics are primarily directed at establishing the existence of a Prime Mover. For that argument to work, all Aristotle needs is that nothing can be an independent self-mover. In a very real sense, Aristotle’s cosmos was full of dependent self-movers: the rotating heavenly spheres, the natural movements of the elements, and the animal soul.
The Analytic Thomist 