The union between mind and body that Descartes posits at the end of Part IV, raises two big worries: (1) what can it mean for two distinct substances to form a union and (2) how can an immaterial substance causally interact with a material one? Many people still consider these worries the biggest obstacle to Descartes' dualistic theory (and thus, in a sense, to his entire metaphysics and physics). Luckily, Descartes' contemporary critics with their modern counterparts and pressed him on these questions in their correspondences. From these correspondences we can arrive at Descartes' answer to these puzzling problems. In his response, Descartes seems to merge these two questions, and to answer them both with an intuition that has been validated by later advances in science and philosophy.

First let us see why Descartes merges the two questions. In order to understand why Descartes does this, it is important to see how Descartes explains the union between mind and body. The best way to describe the union, Descartes claims in IV, as well as in Meditation VI, and in a letter to his friend Regius, is by appealing to the fact that we sense the actions done to body, rather than intellectually perceiving them. When someone else's hand gets burned, we perceive that fact in a very different way than we perceive the burning of our own hand. This is because mind and body are unified.

This way of describing the union seems to imply that the connection between mind and body is causal. To say that the mind and body form a union, it seems, is to say that there is a dense network of causal interactions between mind and body; whenever something is done to body, something happens to mind. In order to answer the first challenge—what can it mean for two distinct substances to form a union, Descartes must answer the second—how can an immaterial substance causally interact with a material one. Descartes answers this question in a correspondence with Princess Elizabeth. There he attempts to challenge the supposition that the only kind of comprehensible interaction is contact interaction, i.e. an interaction in which two material substances come into physical contact, thereby affecting each other. It is perfectly obvious, he rightly claims, that mind and body do interact; we observe this interaction constantly. And because he takes himself to have proven incontrovertibly that mind is immaterial, he believes it must follow that immaterial substances can interact with material substances. The only hurdle to this inference is the flawed supposition that all interaction is contact interaction.

In order to invalidate this supposition, he appeals to a commonly held view regarding gravity. Most people, he claims, implicitly conceive of gravity (which, being pre-Newtonian, he refers to as heaviness) as something distinct from bodies, something that can exist on its own in the absence of body. This, however, involves a conception of a non-extended substance causally interacting with bodies. Though this conception is mistaken (remember that, according to Descartes, gravity is merely a property of body), the intuition present in this mistake—that something immaterial can act on something material—is exactly what is needed in order to defeat the view that only contact interaction is conceivable. We can, therefore, conceive of an immaterial mind acting on a material body and vice versa.

Descartes' intuition—that not all interaction need be contact interaction and that immaterial-material interaction is no more mysterious that material- material interaction—seems to have been largely validated by later advances in science and philosophy. David Hume showed that material-material interactions are not the obvious, well-understood phenomena we take them to be. The evidence we are left with for material interactions is no more than the evidence Descartes insists on for mind-body interaction: we just constantly see it happen. So contact interaction is just as mysterious as material-immaterial interaction. And, in fact, according to modern science, there is absolutely no interaction involving contact between bodies; one body acts on another by way of an electromagnetic field. Descartes' biggest problem, then, is not a problem specifically for his philosophy at all.