So how is matter an interior principle of the real?
The first account of this is to look at the sort of action that we ascribe to matter. If we want to explain why a rock fell, we point to some physical property like its desire for the center of the earth (Aristotle), or its being tugged to the center of mass of the universe (Newton), or a monadic perception of all bodies in the universe that plays out its pre-established nature in their presence (Leibniz) or its bending of space (Einstein). But if we want to explain why the rock fell on the head of an enemy after being carefully rolled off a cliff above him, then any sort of physical explanations like the ones just given become superfluous. Even if one thought that all actions and causes will be given physical accounts – even those seemingly caused by the human will – he still doesn’t think that the explanation of why the rock fell will be in both instances traced back to the matter which is an intrinsic principle of this rock.
Matter is thus an intrinsic principle so far as we can trace some sort of action back to it. All sides seem to agree that the sort of action we are looking for is one that is determined to a result, which seems to mean that it is determined to a result so far as it has idealized circumstances and precisely given initial conditions. Recent theories want to make room for a merely probable determination, but this seems to differ from total determination only by having rougher edges, not by being a different sort of action.
But idealized circumstances, determination to results not yet occurring, and initial conditions all point to matter having to do its work in a system, that is, as a part of a larger whole. If we want to know what the material thing will do, it’s not enough to be told about it, we need to know something about its environment.
But the thing and its environment can only form a single action if one of two states is true:
a.) The thing relates to the other things making its environment as an agent to instruments or
b.) All the things in the environment are instruments to something else.
Now B is just an instance of A when the “something else” is material thing, and so either we posit immaterial agents or B reduces to A. But let’s consider A. This requires that there be some way, in principle, to tell the difference between what acts and what is acted upon. There does not, however, appear to be any such way to tell these apart in a physical system as such. Take the we-trod example of two billiard balls hitting each other. Let’s isolate this as a purely physical system by making it happen in outer space. Did the one hit the other that was resting? Vice versa? Were neither at rest but each moving with velocities that added together to get the result? The question is in principle unsolvable. The example universalizes to matter acting on space, or any other change resolving to local change, which is to say all physical changes.
And so either there are non-material sources initiating action or nothing at all initiates action. Since the second is a dead end, we need some source other than matter to account for the very action that matter performs. This is not to make matter inert or without a distinctive, intelligible contribution, only to say that it is not a sufficient cause.
These non-material sources are either within matter or not. As within them, they are monads or substantial forms. It is not clear at this point that more is necessary to account for the action that defines matter.
But does positing a substantial for within things give us any insight as to which is the agent? Let the two billiard balls each have a substantial form. Does this give us an in-principle way of telling which acts on which? Substantial form has to do more than this to account for action. The substantial form or moand, whatever it might do, needs to make agency identifiable, even in principle. It seems there is no way to do this without introducing self-action within things, thereby giving them, at minimum, what plants have. Accounting for the action of matter therefore requires life as a prior cause.
Matter is thus an interior principle of the real so far as it is capable of being an instrument for life, or to a form making things alive.