Many philosophers, mainly those of a physicalist bent (perhaps non-physicalists also), agree that there is a conceptual difficulty with what is called interactionist dualism (hereafter, ID). This difficulty for ID arises from accepting the following two tenets:
(T1): There are non-physical and physical events of the human person that are ontologically separate.
(T2): The non-physical and physical events of the human person interact causally with one another.
The acceptance of both these tenets leads to the problem of dualistic causal interaction (hereafter, PDCI). As many see it, dualism suffers from a problem of its own making (given its acceptance of the conjunction of (T1) and (T2): namely, how do events of such a disparate nature interact causally with one another? From this one is supposed to infer that they cannot, given the extreme disparity of the things involved. Proposers of the PDCI see this as a devastating conceptual difficulty with dualism!
What is the dualist to do? For one, the dualist could forge ahead by denying that these different events do in fact interact with one other (thus denying (T2). He could offer contemporary versions of parallelism (a la Leibniz), occasionalism (a la Malebranche), idealism (a la Berkeley), or epiphenomenalism (a la Jackson). Although in principle the dualist could offer such accounts, I find them to be both implausible and unattractive. Implausible because they deny a seemingly fundamental datum of experience, namely, my thoughts, beliefs, and desires interact causally with specific physical events to bring about a desired state of affairs. For example, my desire to have a coke and my belief that I can acquire one at McDonald's leads to the event that I go to McDonald's and buy a coke. This just seems true to me. Now these views could be motivated by independent reasons other than to solve the PDCI, but insofar as they are motivated primarily to solve the PDCI, I find them implausible.
Second, and a more plausible response, the dualist could quite forcefully claim that the objector has no metaphysical-cum-logical arguments—other than the proverbial example of the two billiard balls—that would incline him to believe that entities of a disparate nature cannot interact causally with each other. And multiplying examples of how material objects interact causally with one another (like two billiard balls) does nothing to demonstrate that non-physical events cannot interact, as it were, with physical events.
This response is nothing more than to question the very notion of causation that is in play by the objector. What exactly does the objector have in mind when he speaks of 'causation'? Well, at least in the case of Jaegwon Kim, there seems to be the assumption that causation involves the notion of spatial contiguity. But why think that all causation only involves things that are spatially contiguous? Does this not baldly beg the question against the dualist (at least those who accept that the soul is non-spatial)?
Also, the dualist could offer a view of causation in which this problem fails to arise, like the regularity view of causation. On this view, causation is nothing more than an event of one kind that is followed by an event of another. Now the dualist could claim that non-physical and physical events interact causally in the sense that a non-physical event m* is regularly followed by a physical event p*. No interaction problem seems to ensue. Although I would not recommend the defense of such a view, I point this view out merely to show that in order for the PDCI to have its intended sting, the objector must demonstrate that by its very nature, causation, cannot involve non-physical and physical events. And I see no forthcoming metaphysical-cum-logical arguments for this consequent. Moreover, the PDCI does not seem to me to have the force that many philosophers ascribe to it.
