The Contemporary debate over free will is centers on whether the system in which X occurs can give a sufficient account of the actions of X. Those denying free will answer yes, those affirming it say no.
Interestingly, for St. Thomas this way of framing the question did not make for a debate over free will but for a debate over whether life exists, and therefore over whether there is such a thing as soul in addition to body (i.e. soul in addition to physical systems). Life is only really different from the merely physical if there can be self action – i.e if we can isolate some individual from a larger environment or system of interactions attribute an action to it. So does any action come from an individual as such, or does all action occur by the interaction between individual things?
The second thesis, which sees interaction as sufficient, cannot isolate any system from the universe and so it must argue that the universe alone acts. But the only reason the universe is not moved by something else is because there is nothing else to move it. But if there is nothing else, and all actions really are interactive, then the universe ought not to act at all. And so the pan-interactionist thesis, while claiming to suffice to explain action, concludes to its impossibility.
And so the to account for action – even an action that seems purely physical – we need to posit something in addition to individual things qua existing in interactive systems. Soul must enter into the picture somewhere as something that can act without merely interacting. Seen from this angle, the interaction problem, while raising a crucial question, is a colossal missing of the point – we need soul precisely because interaction does not suffice to explain action. The reality of action – so strongly stressed by Blondel and Hannah Arendt, but which has largely fallen from view – requires the reality of something transcending merely physical systems.
There is thus, at minimum, one soul. If we hypothesize that it explains all motion by itself, then we posit a universe soul and speak of the universe alone as alive. All else is alive only as organs of the universe-zoön. But if I decide that I act – even though this self is clearly conditioned by interactive systems and perhaps even by the animate universe – then I too must be ensouled. I see no rational way to allow soul to the universe and deny it to myself – I allow soul so far as I am convinced anything is alive, and while I can make sense of denying life altogether I can make no sense of allowing it to be real to something other than myself while denying it to myself.
This is really a corollary to Arendt’s claim that to act is to be an origin of something and therefore for there to be something new in the universe. This requires that the physical universe be an instrument of the living. It is arguably a necessary instrument, i.e. the universe is to all action what a saw is to sawing. On this account, the actions of living individuals, while transcending the physical, could not exist without it, and so while no soul interacts with matter it nevertheless cannot exist without it.
Just Thomism 
BM said,
June 24, 2014 at 10:01 am
How much of or in what way is your thinking here a response to the difficulties of Newton’s third law? All talk of action as necessary to explain motion as opposed to mere interaction reminds me of this, but you never mention it. At least not here.
James Chastek said,
June 24, 2014 at 10:36 am
I think the Third Law is a description of what action is in physics. It requires that physics is a study of systems. The Third Law, importantly, was conserved in the transition to contemporary physics and appears to be fundamental to a scientific account of the purely physical.