Substance dualists believe that humans are comprised of two radically different types of substances that interact but can, in principle, exist independently from each other. One popular version of substance dualism maintains that the mind is a non-physical, simple (not made of parts), and non-spatial substance whereas the body is a physical, complex (made of parts), and spatial substance (for my preferred argument in defense of this view, go here). One of the most powerful objections to this view focuses on the interaction of the substances itself: how can two radically different types of substances interact at all? Two pieces of matter seem capable of interacting. But how can one piece of matter interact with something like a mind that has no mass at all? How can an extended body be causally related to something that is non-extended and is not even in space at all? It doesn’t seem possible. Perhaps then it is better to argue that mental states are actually physical states. In doing so we can hope to account for the fact that our minds do indeed have physical effects.
Substance dualists, however, maintain that identifying mental states with physical states will fail to account for aspects of our mental life such as qualitative experience, rational inference, intentionality, moral values, free will, and unity of self over time (among other things). Indeed, they argue for substance dualism based on some or all of these things. They therefore seek to maintain dualism and face the interaction problem.
One way to handle the problem is to focus on the mysterious nature of causality itself. The nature of causality has been philosophically problematic at best and an article of faith at worst since David Hume’s devastating critique of it from an empirical perspective (see my overview here). But if this is the case can issues related to causal interaction be decisive against substance dualism if causality is so problematic for everyone?
And it is not just Hume’s critique we have to worry about. For quantum mechanics appears to show us that we can have knowledge without any physical causality. James Robert Brown, in his book The Laboratory of the Mind (New York: Routledge, 1991), helps us approach an understanding of this point:
“Einstein, Podolsky, and Rosen (1935) set out an ingenious argument to undermine the then reigning interpretation of quantum mechanics, the Copenhagen interpretation, which holds that quantum mechanical systems have their properties only when measured. EPR attacked the idea that measurements create instead of discover….We start with a coupled system which can be separated, say an energetic particle which decays into a pair of photons moving in opposite directions. The initial system has spin 0 and spin is conserved. If we make a spin measurement on one photon along, say, the z axis, and get what is commonly called spin up, then a measurement on the other photon along the same axis will result in spin down. We further suppose that the two measurements are done far from one another, indeed, outside each other’s light cones. This last requirement is sometimes called the locality assumption. According to special relativity, the upper bound on the velocity of any signal prevents the measurement of one photon having an effect on another” (71).
It turns out that we know, through scientific experimentation, the outcome of the distant measurement. But how? EPR argued it can’t be the case that our measurements of one photon create the state of the other photon since there is no causal connection between the two (no light gets from to another). Rather, it must be the case that both photons had their states already which we then discover, rather than create, with our measurements. EPR thought this argument shows that there must be some hidden variables, that is, some other unknown physical phenomena besides the phenomena presented in quantum mechanics. Einstein claimed “God doesn’t play dice with the universe” and he was hoping his critique of the Copenhagen interpretation would take steps towards discovering a deeper, more fundamental level of reality.
But this hope was eventually found to be hopeless when, in 1964, John Stewart Bell discovered an inequality—Bell’s Inequality—which showed there cannot be any hidden variables if Special Relativity holds. Again, consider Brown:
“J.S. Bell generalized the EPR argument, and subsequent experiments based on his results have shown that the EPR conclusion is hopeless…On the assumption that such hidden variables (i.e., a common cause) are present and that locality holds, Bell was able to obtain an inequality that now bears his name. The remarkable thing is that his result has straightforward empirical consequences; it can be put to the test, something which we could never do with EPR. The outcome is not a happy one for so-called local realism. As long as we hold to special relativity—and we should certainly do that—then we must disavow hidden variables. In any EPR-type situation there is no common cause of the correlated measurement outcomes” (73).
But this would mean we have knowledge without a physical causal connection in one of the most important experiments in physics. Brown offers an interpretation of this fact: “The moral for the causal theory of knowledge [the view that knowledge presupposes the knower has a physical causal connection to that which she knows] is simple: it’s wrong. We have knowledge of an event; but we are not causally connected in any physicalistic way to that event, either directly (because of locality) or through a common cause in the past (since there are no hidden variables)” (152).
As already mentioned, substance dualists try and account for aspects of our mental life such as qualitative experience, rational inference, intentionality, moral values, free will, and unity of self over time. Typically they contend that these aspects of our mental life have correlates in the physical world just as physicists claim a measurement of one particle is correlated with another particle. But if modern physics shows there are times when we cannot, on the one hand, establish a physical causal relationship between two phenomena and, on the other hand, have knowledge of how these two phenomena are correlated, then why should substance dualists be required to provide a physical account of how mind and body are correlated? To be sure, a substance dualist need not stop with this negative response to the problem. There have been many positive responses to the problem of interaction that attempt to show how mind might indeed interact with matter (for an overview, go here). But the above appeal to the mysterious nature of causality might at least show that if interaction is a problem for a substance dualist then it is also a problem for a naturalist who accepts, among other things, Einstein’s Special Relativity and Bell’s experiment.