The interaction problem is often taken as a problem of how an immaterial mind could move a material body, but it is pretty clearly subset of a general claim that the immaterial cannot explain the action of material things. Any evaluation of the claim pretty clearly involves some analysis of interaction: specifically, if some agent interacts with a physical being, is it necessary for that agent to be acted on physically? This certainly seems to be the case: if the physical action is pressure, for example, this requires just as much the pressing of the agent on the object as the object on the agent. Similar considerations apply to work, force, torque, resistance, and any other dynamic reality that a physicist might study.
To start at a level everyone can agree with, the interaction problem is a problem about interaction, that is, about action between things. The action of me thinking is therefore not an interaction, even if it presupposes some interactions, since thought is not the sort of action that goes from one thing to another as an interaction. Interaction is essentially transitive action, specifically, it involves both A acting on B, and B acting on A. But is there a real difference between action and interaction? There clearly isn’t any sort of separability of the two when A and B are bodies: in fact, it’s hard to see what physical action could be without interaction: I can’t throw something unless it resists my hand in some way (which is what “gripping” something amounts to) and this grip pressure can be measured just as easily in the ball as in by hand. Action between physical objects presupposes interaction.
This point can be made more general than just physical objects, for even if we posit some non- physical agent, there can still be some co-dependence on the physical thing. Transitive action by definition goes out of the thing that acts, and thus this action must be in another – even an angel depends on the thing its action is in. Interaction is thus a species of a larger genus, sc. co-dependent being. But there is, however, a pretty clear exception to this, namely, if an agent not only acted on X, but was responsible for its very existence. In this case, there would not be a co-dependence of the agent on what it moved. It is ridiculous to say some action is dependent on X if the existence of X depends on that action. Thus, the agent’s co-dependence on the mobile only happens to the extent that the mobile does not depend on the agent for its existence, and since interaction is a kind of co-dependence, interaction can only occur to this limited extent. Briefly, to the extent that A causes the existence of the B, A can act on B without interacting with it.
But it is exactly the claim of those who speak about the soul moving the body that the soul is the cause of the existence of the body. This is why all who have spoken of the soul insist that a corpse is no more human than a statue. A body is one thing and a corpse is not. The soul thus really does act on the body, but its action is not interaction. This is not anything proper to human beings, but to anything living. One really can speak of the actions of the soul on the body, but this is not a kind of action that involves interaction, since there is no separable existence of the body as a body (whether there is a separable existence of soul, and for what, is another question.) But all that is important to note here is that the action of soul and body is not an interaction- or at least not to the extent that soul is responsible for the existence of the body.
But while the soul is responsible for the existence of body to the extent that the body is its own and of certain type, it clearly isn’t responsible for its existence in absolutely every way. Existence as such is not referred to the soul, but to what exists by its very nature. For such a being, it is also the case that action does not involve interaction, and this in an absolute sense.
To sum up, interaction is a kind of co-dependence. All physical objects interact with each other since their actions involve this co-dependence: viz. pressure, force, work, torque, friction, etc. all require the interaction of physical things, and there can be no such actions without interaction. But there can be an exception to this, even in the physical order, when that which acts is responsible for the very existence of what it acts upon – since in the measure that A is responsible for the existence of B, it makes no sense whatsoever to talk about A depending on B, and thus is makes no sense to speak of it interacting with it, or it at least makes no sense for its action to be the sort of action that is essentially interactive: pressure, force, etc. Seen in this way, physical action always bespeaks ontological degradation, for it is a failure to cause something on the level of its existence, and, where the agent itself is acted on physically, there is the very bottom level of ontological degradation.