Did the Council of Vienne Rule Out Substance Dualism?

I recently received a message from a reader, asking whether the Council of Vienne rules out the view in philosophy of mind known as "substance dualism." According to substance dualism, the soul is an incorporeal substance which can exist separately from the body. Famously, a version of substance dualism was held to by Descartes.

I thought the response I wrote might be of interest and help to some readers. I would argue that, whatever other issues one might have with substance dualism, this position is not ruled out dogmatically. (This is not to say that I endorse the position or that I even consider it reasonable, of course.)

The ratified decree of the Council of Vienne, promulgated by the Apostolic See, says this:

Moreover, with the approval of the said council, we reject as erroneous and contrary to the truth of the catholic faith every doctrine or proposition rashly asserting that the substance of the rational or intellectual soul is not of itself and essentially the form of the human body, or casting doubt on this matter. In order that all may know the truth of the faith in its purity and all error may be excluded, we define that anyone who presumes henceforth to assert defend or hold stubbornly that the rational or intellectual soul is not the form of the human body of itself and essentially, is to be considered a heretic.

Here is the answer I gave to the questioner, which I've adapted from our exchange:

"It has been a while since I investigated this, but yes, I was pretty convinced at the time that the condemnation does not rule out substance dualism. Here are a few reasons:

1) The text itself just says that the intellectual soul is per se and essentially the [substantial] form of the human body. That's what it says. That statement is not equivalent to the statement that a substantial form cannot be a substance. Someone might personally think that follows, because of a certain metaphysics of substantial forms. But the Council itself does not assert that.

(Incidentally, as far as the metaphysics goes, Aristotle himself says that the form is a substance, in Metaphysics. The interpretation of these texts is quite convoluted of course, but it's clearly there. I'm not saying Aristotle was a substance dualist, of course.)

2) The Council primarily was aiming to condemn Peter John Olivi, who denied that it was the intellectual soul that was the form of the body. (See here.) Thus, if you hold to a theory of conciliar decrees that would primarily take a Council's intentions rather than a Council's texts as authoritative (a position I very much disagree with), that would place the emphasis of the Council's condemnation on the "intellectual soul" part, and the target of the condemnation would be Olivi, not substance dualism. In short, it is anachronistic to read a condemnation of substance dualism into the Council's decrees.

3) The view that the soul is a substance continued on among Catholic scholastic thinkers whose orthodoxy was impeccable. In particular, Suarez thought that both the soul and the body were substances (albeit "incomplete" ones, as he says). Nobody ever thought this was particularly controversial, because again, one can believe this while still believing the intellectual soul is the substantial form of the body.

4) Even as thoroughgoing a substance dualist as Descartes was completely explicit that he held the soul to be the substantial form of the body. (He says this in several letters.) There are some good books on this, like Justin Skirry's Descartes and the Metaphysics of Human Nature. As a side note, I generally agree with those quasi-revisionist interpreters of Descartes like Marjorie Grene and especially Roger Ariew who think Descartes should be understood within the context of late-medieval metaphysics.

Apart from Descartes, J.P. Moreland has published a book that strongly advocates for both views: that the soul is the substantial form of the body, and that it is a substance. (How authentic to Thomas it is, I'm not sure.)"

The reader also kindly pointed me to Pohle and Preuss's commentary on the dogmatic decree, here. Here was my response:

"On pp. 148 ff. the authors are quite cautious in noting that this text should not be interpreted as saying much about hylomorphic metaphysics, and if I understood right they even suggest that it is compatible with Atomism, which goes well beyond anything Descartes ever said! Given their interpretation in 148 ff., I cannot see how Descartes's position could even remotely be thought of as being within the purview of the Council's condemnation.

They are also correct to point out that the condemnation was drawn up by Scotists, who give the soul and body a rather stronger form of existence than Thomists in their account of the body-soul composite. It would seem especially absurd, in that case, to think they were somehow endorsing a specifically Thomistic hylomorphic metaphysics."

Again, my point here is purely one of what is Church doctrine. There are many issues with substance dualism as a philosophical position, and probably the Church's formulation rules out some versions of it, e.g., if you really did take seriously the idea that the soul is just a "ghost in the machine." But no real historical advocate of substance dualism would have said that, to my knowledge (certainly not Descartes). Arguably, Plato was a substance dualist, and clearly held that the soul is the form of the body that makes it to be living. Probably some Platonic Christian philosophers prior to the revival of Aristotelianism held similar views. A priori, then, it seems highly unlikely the Council was trying to rule these out. If they are to be ruled out, I think it would have to be for metaphysical reasons.