Challenging the Interaction Objection to Substance Dualism

Is this objection as good as it is taken to be?

In this post, I noted that substance dualism (SD) faces the interaction problem (IP). How can two radically different realms (the material/physical and the immaterial/mental) interact? For example, according to SD, my mind is not a material entity and my body is a material thing. Yet I can mentally direct my physical hand to handle a cup of coffee; the physical coffee brings about changes in my mental states if I drink it. How does that work? How can something in the material camp cause events in the immaterial camp and vice versa?

The IP is an important issue. Some take the IP to be an insurmountable objection to SD. I see at least three reasons to be cautious with this pronouncement.

First, the IP presupposes that for one thing to interact with another, the two relata must be relevantly alike, i.e., that pertinent likeness is a necessary condition for interaction. Let’s call this the principle of interaction between likes (PIBL).

Now, I hold that the like-cases principle (LCP) in ethics and law is reasonable: we ought to treat relevantly alike cases alike and different cases differently. But why should we accept the PIBL? I haven’t seen much in the literature to support this principle.

Moreover, there seem to be counterexamples to the PIBL. Take the property of redness. Say it’s an abstract object and a universal. And yet it interacts (broadly speaking) with concrete objects such as this apple and that cherry. Abstract objects are quite different from concrete ones, and universals are distinct from particulars.

One might respond that the IP should be construed more narrowly as a matter of causal interaction.* Fair enough.

How about these counterexamples?

— the knocking of one’s knee against the corner of the table causes a qualitative sense of pain in the mind of the owner of the knee;

— the eating of the apple pie causes a qualitative sense of sweetness in the eater’s mind;

— the qualitative state of dread causes a physical trembling in the one who dreads.

— the qualitative joy of the victorious athlete generates a celebratory fist pump and sounds of elation.

It seems clear that the physical knocking of the knee is not relevantly like the what-it-is-like to be in pain. Yet it is quite reasonable to believe that the former interacts with the latter. Ditto for the pie, dread, and athlete cases.**

Second, suppose arguendo that the PIBL is true. What if the immaterial/mental and the material/physical are both real and relevantly alike, but not alike in the manner we assume they should be? Do we know enough about the physical (or the mental, for that matter) to conclude dispositively that the physical and mental are not relevantly similar? I doubt it.

Third, causal applications of the IP (e.g., the baseball bat hitting the baseball) seem to take for granted that energy must be wholly physical. Why think that? Why can’t there be nonphysical/immaterial energy such that the mental energy of a mind can move the physical hand to reach for the coffee cup? Some might worry that this sounds too mysterious. But do we understand energy and matter well enough to conclude with epistemic certainty that energy must be wholly physical? Are we justified in claiming to possess conclusive knowledge of such matters? I don’t think so.

There is much more to say. Nevertheless, at this point, it seems there is room for reasonable doubt regarding the efficacy of the IP.

*One could deny that the causal IP is a problem by denying that mind and body causally interact. I’ll ignore this move for now because it would take us into positions such as occasionalism and pre-established harmony which I can’t discuss in detail here. One could also hold that mind and body interact, that this is a fundamental fact, and that there is no explanation for it. This claim might be taken ontically (i.e., there is no explanation at all) or epistemically (i.e., there is no explanation available to the human mind).

**Some simply deny that qualia exist. I won’t discuss this view. For me, qualitative experience is beyond reasonable doubt.