"How can the mind move the body?" This is one of the most familiar rhetorical questions for dualists about the mind. It is rhetorical since it is meant to make the point that dualists can't explain how the mind moves the body, and that this is a problem for dualism.
One problem is that this question is highly ambiguous. This is reflected in the literature by the fact that the phrase 'interaction problem' is used by different philosophers to refer to quite different arguments. When someone poses this question there are multiple objections someone could have in mind. Someone could be saying it's in principle impossible for dualists to provide an explanation. Or they could simply be saying there is no plausible candidate for an explanation. Or they could be saying it's very likely that non-physical causation never happens in our world. Moreover, they could pose these arguments for different reasons. So it's important to get clear on which objection we're talking about. I'll try to distinguish a few ways we can formulate this objection and then reply to these objections. In the first post I'll identify and respond to what I call the 'Different Substances Objection'.
#1: The Different Substances Objection
First, one might object that, since mind and matter on a dualist view are completely separate there is no way for them to interact. Everything is mental or material, but not both. So there could not be interaction between the two. The problem is that it's not generally true that entities from two mutually exclusive categories cannot interact causally. Everything is either a proton or a non-proton for instance, but that doesn't mean protons and non-protons cannot interact.
Another very similar form of this objection is that mind and matter are different types of substances, so the two cannot interact. Or a more particular version of this objection would say that the mind is non-extended and the body is extended, so the two cannot interact. One problem is that, in general, it is false that in order for substance x to interact with substance y of kind F, x must itself be of kind F. You don't need to be a human to interact with a human; you don't need to be a proton to interact with a proton; etc.
A final and more sophisticated way to formulate this version of the objection is that since mind and matter share no intrinsic properties, the two cannot interact. First of all, the main premise of this argument is not true. Being a substance is common to both our minds and our brains. Having causal powers is intrinsic to both (note: one need not presuppose causation between mind and body occurs to hold this is true, since one should at least admit mental to mental causation). They both have the property of having metaphysical components. Of course, they don't share any intrinsic physical properties (since the mind does not have physical properties). But at least some of what I have listed are plausibly intrinsic properties.
More importantly though, why do two substances have to have common intrinsic properties to causally interact? This principle would have to be motivated by some more general theory where two substances' having common intrinsic properties P1, ... , Pn plays a relevant role in their ability to engage in causal interaction. In particular, to be relevant, if x causes change C in y in some way, then these P1, ... , Pn must contribute some causal influence to C. I'm not sure how else one would motivate the main premise here.
But we can imagine situations where the common intrinsic properties of agents plays no relevant role in the interaction of the two. For instance, even though a Jedi and a table share the intrinsic properties of having mass or volume or other physical properties, when the Jedi uses the Force he doesn't in any way seem to take advantage of his having mass or volume or his other physical properties. Of course, this is just an imaginary situation, but it seems perfectly coherent and thus there can be no a priori reason for thinking common intrinsic properties must play a relevant role in all causal interaction (of course, this isn't to deny that sometimes they do).
In fact, there might be some cases which are actual counterexamples. For instance, consider the EPR paradox cases from quantum mechanics. Suppose you have a source emitting an electron-positron pair in a state of quantum entanglement, where the spin of each is anti-correlated with the other. In other words, if electron e has upward spin then p has downward spin, and if e has downward spin then p has upward spin. Suppose moreover you have two observers A and B in different locations who can measure the spin of the particles along some axis Z, and e is sent to A while p is sent to B. If A measures e as having an upward spin then B will measure p as having a downward spin with 100% probability. On the other hand, if A measures e as having downward spin, B will measure p as having upward spin with 100% probability. Since experimentation and Bell's Theorem rule out local causal explanation here, and supposing there is causal interaction between e and p, there must be some causation here where local intrinsic properties such as mass, volume, velocity, etc. do not play a role in the causal interaction.
Of course, the particles do share the intrinsic properties of having spin. But it's not e's simply having spin which contributes causal influence to p's particular spin; rather, it is e's having an upward or downward spin which does so. For what the spin of p is depends on the actual spin of e. So it isn't clear that we can identify some common intrinsic property of e and p where's e's having this property causally influences p's having a particular spin.
This seems to me to be an adequate reply to the different substances objection. In the next post I'll talk about what I call the 'No Mechanism Objection', which poses the problem that there seems to be no familiar model which could make causal interaction between the mind and the body intelligible.