Those who reject dualism (the view that man is made up of two kinds of substances: physical and immaterial) often cite the “interaction problem” as an argument against the view. Stated simplistically, the interaction problem is to explain how an immaterial entity such as a mind/soul could causally interact with material entities. One envisions the Hollywood movies in which a ghost is desperately trying to pick up a beverage or kiss someone to no avail. Try as he might, he cannot connect his immaterial self to the material world to affect it in any way (unless you are Patrick Swayze!). Many monists think the interaction problem alone is sufficient to dismiss dualism as a possibility.
Such an approach to the question seems wrongheaded, however. One should not look at the queerness of mind-body interaction and immediately conclude that the mind cannot exist independent of the brain. One must first evaluate the evidence for the existence of such an entity. If there are good, independent reasons to think the mind is not an immaterial entity—but can be reduced to the brain or arise from material processes—then the interaction problem could serve as further confirmation that there is no soul. But if there are good reasons to think the mind is an immaterial entity separate from the brain, then the interaction problem—while difficult or even impossible to explain—is insufficient to overturn the evidence that the mind is immaterial. While we may not know how the mind interacts with the material world, we know the two entities do exist, and do interact with each other. One need not explain how something occurs to know that it occurs. We may forever be ignorant of how the mind and body relate to each other, but we have direct awareness and experience of the fact that they do.
To say “Because I cannot conceive of a way in which the soul could causally interact with the physical realm, the soul cannot exist” is like saying “Because I cannot conceive of a means by which voices can be transported thousands of miles in a split there can be no such things as telecommunication.” But we know telecommunication is possible because we use it all the time. We know phones exist! We do not need to know how telecommunication works to know that it works. Likewise, all of us have an immediate intuition that we are not identical to our bodies. Any inability I may have to understand or explain the interaction of my self with my body does not count against the fact that I have a self that is distinct from and interacts with my body.
While there may be good reasons to reject dualism, the interaction problem is not one of them. Indeed, some theologians and philosophers have proposed solutions to the problem. Whether those solutions are sound or not can be debated, but what should not be debatable is the means by which we determine if the mind exists. And as a matter of method, our answer to the how question must be secondary to the evidence for the what question. If there is good reason to believe the mind is immaterial, then we should do so even if we do not know how the mind interacts causally with the material world.