You can find the relevant letters in several anthologies, such as You can also find them . What follows is a summary of the key points in their back-and-forth, with some comments. (which is the source from which I’ll be quoting).
Elisabeth begins by noting that if we think of efficient causation on the model of one extended object making contact with another and pushing against it (which would have been natural for those working with the then-ascendant Mechanical Philosophy’s conception of matter), then the interaction between soul and body – when conceived of in Cartesian terms – is hard to understand. For whereas Descartes takes the body to be pure extension, he takes the soul to be pure thought devoid of extension. Elisabeth suggests that clarification requires an account of the nature of the substance of the soul, apart from its activity of thought.
Descartes responds by saying that our notions of the soul, of the body, and of the union of the two are each primitive, and that we must be careful not to attribute what is true of one of these to the others. Now, this is what happens when we try to conceive of the manner in which the soul moves the body on the model of the way in which one physical object moves another. This is, as philosophers would say today, a “category mistake,” and it seems that Descartes is implying that Elisabeth is guilty of such a mistake in formulating her puzzlement over the nature of soul-body interaction.
Descartes also suggests that such a category mistake is committed by the Aristotelians of his day who thought of physical objects as, by virtue of their weight, drawn toward the center of the earth – where it seems that it is the teleology or final causality posited by this account that he has primarily in mind. He says that this account wrongly attributes to a physical object’s relation to the earth’s center what in fact holds of the body-soul relationship. His point seems to be that while the notion of teleology has (in his view) no application in physics, it does provide us with a way of understanding how the soul acts on the body.
Descartes’ reply to Elisabeth is not entirely unhelpful. Certainly he is right to emphasize that he would not say, and is not forced by his commitments to say, that soul-body interaction is correctly modeled on efficient causation between two physical substances. However, his reply is not entirely helpful either…
In her response, Elisabeth suggests that it is not clear how the weight analogy provides a better model by which to understand how the soul moves the body. And Descartes’ explanation is indeed not entirely lucid, though one can imagine ways to develop it that Elisabeth does not consider. Presumably the analogy goes like this: On Aristotle’s model, the center of the earth is the end toward which a physical object moves by nature; and on Descartes’ model, the soul is the end toward which the body moves by nature. And though Descartes rejects the teleological account of the movement of physical objects relative to the earth, teleology does provide a way of modeling the movement of the body relative to the soul.
In any case, though Descartes himself doesn’t explicitly say all that, it seems to me to be a way of interpreting him that accounts for why he would think the weight analogy at all helpful. But immediately, problems arise. Part of the point of Descartes’ adoption of a mechanistic conception of matter was to get teleology out of it. But now he seems to be putting teleology back into matter again, at least for the purpose of solving the interaction problem if not for purposes of general physics. Isn’t this ad hoc?
Elisabeth also still regards the case of soul-to-body causation as so puzzling that she says she finds it “easier… to concede matter and extension to the soul, than the capacity of moving a body… to an immaterial being.” This is in my view her main mistake, for reasons I will explain presently. But she immediately makes another point which is correct, extremely important, and widely neglected. For it is not just the soul’s capacity for moving the body that Descartes has to explain, but also its “capacity… of being moved” by it. She continues:
It is, however, very difficult to comprehend that a soul, as you have described it, after having had the faculty and habit of reasoning well, can lose all of it on account of some vapors, and that, although it can subsist without the body and has nothing in common with it, is yet so ruled by it.
End quote. In short, whatever one thinks of soul-to-body causation, causation in the other direction – that is to say, body-to-soul causation – is really mysterious if one accepts a Cartesian account of soul and body. And the reason why it is so mysterious is, in my view, linked to the reason why soul-to-body causation is not in fact as problematic as Elisabeth thinks it is, or at least not problematic in the specific way she thinks it is.
But I’ll come back to all that too in a moment. First let’s consider the rest of the exchange. Descartes’ response is once again to comment on the differences between our notions of the soul, of the body, and of the union between them. He appeals to the traditional distinction between the intellect or understanding, the imagination, and the senses. He says that the soul is properly known by the understanding alone, the body by the understanding together with the imagination, and the union between them by the understanding together with the imagination and the senses.
The idea here seems to be that since the soul is, as Descartes understands it, pure thought devoid of extension, it is the abstractness of purely intellectual apprehension by which we most accurately understand it. The body qua extension, however, is best understood by the intellect together with the sort of mental imagery we entertain when doing geometry. And the closeness of the union between soul and body is evident in attributes which, in Descartes’ view, neither soul nor body can have on its own – namely, appetites, emotions, and sensations, which he takes to be hybrid attributes of a kind that exist only insofar as a res cogitans and a res extensa get into a causal relationship. Hence, Descartes seems to be saying, we need to rely on our experiences of bodily sensations, affective states, and the like – and not just on intellect and imagination – properly to understand the causal relationship between soul and body.
How is this an answer to Elisabeth? Descartes’ point seems to be that the causal relation between soul and body seems mysterious if we rely on the intellect alone, or on the intellect together with the imagination, in order to understand it – but that it will be less so if we rely on the senses too.
The suggestion is certainly interesting, but that doesn’t mean that it is, as it stands, compelling. If Descartes meant only that, as a matter of phenomenology, the close relationship between soul and body seems perfectly obvious and natural to us, then he would certainly be correct. But of course, that isn’t really what Elisabeth is asking about. Her question is not about whether soul and body seem to us to interact, but rather about how they could in fact do so given what Descartes claims about the natures of each.
Elisabeth’s last word
Though Elisabeth and Descartes exchanged other letters in later years, in this particular exchange on the interaction problem we only know of one further letter, which is from Elisabeth – to which, as far as we can tell, Descartes did not reply. She was not convinced by his answer, for exactly the reason I mentioned. She writes: “I too find that the senses show me that the soul moves the body; but they fail to teach me (any more than the understanding and the imagination) the manner in which she does it” (emphasis added).
Her own proposed solution is to suggest that “although extension is not necessary to thought, yet not being contradictory to it, it will be able to belong to some other function of the soul less essential to her.” In other words, she proposes that soul and body can interact because the soul has, after all, extension as one of its attributes, and by means of it can cause changes in and be affected by the body in the same way that any two physical objects interact.
This is an interesting proposal that amounts to a version of what is these days called property dualism, but of a very different kind than the sort usually on offer today. Contemporary property dualists suggest that a material substance, the human body, can have both physical and non-physical attributes. What Elisabeth is suggesting is that an immaterial substance, the soul, might have both physical and non-physical attributes.
But there are two problems with this idea considered as a solution to the interaction problem facing Descartes. First, it turns out that even body-to-body interaction is not as unproblematic as Elisabeth (and most other people who comment on the interaction problem) assume. For Descartes’ abstract mathematical conception of matter is so desiccated that it is hard to see how it can have any efficacy at all with respect to anything, whether physical or non-physical. Occasionalism – attributing all causality to God rather than to anything in the created order – was a natural position for Cartesians like Malebranche to take, and Descartes himself arguably took it with regard to everything except soul-body interaction.
A second problem is that if you are going to attribute physical properties to the soul in order to explain how it interacts with the body, why not go the whole hog and make the whole body itself an attribute of the soul? That way you don’t have to posit any interaction between soul and body at all, because they will no longer be distinct substances.
Indeed, you’d be very close to returning to precisely the Scholastic conception of soul and body that Descartes was trying to replace. You’ll be treating a human being as one substance, not two, but a substance with both incorporeal powers (thinking and willing) and corporeal ones (seeing, hearing, digesting, walking, etc.). And I would say that that is indeed the correct solution to the interaction problem: to dissolve it by giving up the Cartesian thesis that soul and body are distinct substances, so that there aren’t any longer two things that need to “interact.”
As I have often suggested, the real problem with Descartes’ position is not that he has trouble explaining how soul and body interact. The problem is that he thinks of them as interacting in the first place. It is that he posits two substances rather than one. And the reason this is a problem is that he thereby simply fails to capture the truth about human nature. For his model makes of the soul something like an angelic intellect, and the body merely one physical object in the world alongside others that an angel might push about, the way that a demon pushes about an object that it possesses (as in the case of the Gadarene swine). It makes of the body something entirely extrinsic to us (though this was certainly not his intent).
This is the force of Gilbert Ryle’s famous characterization of Descartes’ position as the theory of the “ghost in the machine.” The problem isn’t: “How does an immaterial substance have any effect on the body?” That’s no problem at all for something immaterial – after all, God and angels do it, as both Descartes and Elisabeth would have agreed. The problem is rather: “How, if soul and body are two independent substances, can the soul affect the body in the specific way that it does (rather than in the way a ghost or an angel would)?” The problem is explaining how the body could be a true part of you rather than a mere extrinsic instrument that is no more part of you than any other physical object.
Elisabeth was mistaken, then, to make a big deal of the question of soul-to-body causation as such. Of course, it is easy to think such causation mysterious if you model all causation on push-pull causation between physical objects, but as Descartes rightly says, it is a mistake to do that. (To be sure, it helps if you’re looking at the issue in the light of the complex theory of causation that the Scholastics had developed – and which Descartes himself was familiar with and had not entirely abandoned, as later modern philosophers would.)
Here’s a geometrical analogy that might be helpful. (It’s only an analogy. I am not saying that immaterial substances are higher-dimensional objects.) Consider two-dimensional creatures of the kind described in Edwin Abbott’s Flatland. They might find talk of three-dimensional entities quite mysterious and not understand how such things could possibly “interact” with their own world. But in fact, of course, a three-dimensional entity generating effects in a two-dimensional world would be no problem at all. Similarly, while we lapse into thinking in terms of a crude push-pull model of causation and wonder: “Gee, how could an immaterial substance have any effect on matter? It’s so mysterious!”, the angels and demons look on thinking: “Seriously? How pathetic.” As the Scholastics would say, immaterial substances exist and operate at a higher ontological level than we do, not a lower one. And higher orders have no difficulty affecting lower ones. We might find it mysterious how they do so, but that is unremarkable considering that our cognitive faculties are primarily geared toward understanding the lower, material order.
What would be truly problematic is a lower order affecting a higher one. For example, a two-dimensional entity would have great difficulty having any effect on a three-dimensional one, at least if the three-dimensional one doesn’t cooperate. And in an analogous way, it is mysterious how a purely material substance could have any effect on an immaterial substance.
This is why Elisabeth’s point about body-to-soul causation is so important. If soul and body are two distinct substances, then even if the soul could, as a substance of a higher ontological order, produce effects in the body (even if only in the way an angel might), it is nevertheless entirely mysterious how the body could produce effects in the soul (any more than a stone or a tree could have any effect on an angel or demon).
This problem does not arise for the Scholastic conception of soul and body, because, again, it does not regard them as distinct substances in the first place. A human being is one thing, not two, albeit a thing with both corporeal and incorporeal activities. And since it is one thing, the question of interaction does not arise.