Thomas Aquinas, following Aristotle, views the soul as the form of the body. Anima forma corporis. Roughly, soul is to body as form is to matter. So to understand the soul-body relation, we must first understand the form-matter relation. Henry Veatch points out that "Matter and form are not beings so much as they are principles of being." (Henry B. Veatch, "To Gustav Bergmann: A Humble Petition and Advice" in M. S. Gram and E. D. Klemke, eds. The Ontological Turn: Studies in the Philosophy of Gustav Bergmann , University of Iowa Press, 1974, pp. 65-85, p. 80) 'Principles' in this scholastic usage are not propositions. They are ontological factors (as I would put it) invoked in the analysis of primary substances, but they are not themselves primary substances. They cannot exist on their own. Let me explain.
An ordinary 'sublunary' particular such as a man, a horse, a tree, a statue, a 'primary substance' in Aristotelian nomenclature, is a this-such. The thisness in a this-such is the determinable element while the suchness is the determination or conjunction of determinations that determines (delimits, characterizes, and informs) the determinable element. Veatch's point is that the determinable element cannot be an ontological atom, an entity more basic than the composite into which it enters; it is not an ontological building block out of which, together with other such 'blocks,' the this-such is constructed. The determinable element cannot be a basic existent; it must be a principle of a basic existent, where the basic existent is the this-such. And the same holds for the determining element, the form.
This implies that what is ontologically primary is the individual substance, the this-such, which entails that matter and form in an individual substance cannot exist apart from each other. They are in some sense 'abstractions' from the individual substance. They are nonetheless real ontological factors, as opposed to theoretical posits having a merely mental being; they cannot, however, exist on their own. They are not themselves substances. That is what I mean by my use of 'abstractions.'
But what exactly is a (primary) substance?
A substance is a thing to which it belongs to be not in a subject. The name 'thing' (res) takes its origin from the quiddity [quidditas = whatness], just as the name 'being' (ens) comes from 'to be' (esse). [Ens is the present participle of the infinitive esse.] In this way, the definition of substance is understood as that which has a quiddity to which it belongs not to be in another . (Aquinas, Summa Contra Gentiles, Bk 1, Ch 25)
The form and the matter of a material substance, then, are are not themselves substances because it does belong to them to be in another, namely, in the substance of which they are the form and the matter. Hylomorphic dualism is not a dualism of substances. Here we appear to have an important difference between Aquinate and Cartesian dualism. But the difference may be less than at first appears.
Now the form in a material this-such is not merely tied to matter in general, in the way that Bergmannian first-order universals are tied to Bergmannian bare particulars in general; the form is tied to the matter of the very this-such in question. This is because Aristotelian forms are not universals. And the same goes for the matter: the designated matter (materia signata) of Socrates is tied to the very form that is found in Socrates: that parcel of matter cannot exist apart from Socrates' substantial form. The two ontological factors (as I call them) are necessarily co-implicative. Neither can exist without the other. The two together constitute the individual substance, which is being in its most basic sense. For Aristotle, "being is said in many ways," to on legetai pollachos, and the most basic sense is being as substance (ousia). Aristotelian ontology is ousiology. (Aristotle, Metaphysics, 1028b4)
I said that hylomorphic dualism is not a dualism of substances, and thus that it appears that soul-body dualism in Aquinas is very different from soul-body dualism in Descartes. But the picture is complicated by the Thomist doctrine that the souls of rational animals, unlike the souls of non-rational animals, are subsistent. What this means is that human souls are capable of existing in a disembodied state. This capacity is exercised at death when we humans shed our bodies. We continue to exist as disembodied souls and thus as matter-less forms. But now a tension, if not a contradiction, comes into view. It is not clear how all of the following propositions can be true:
a) The form of a material substance is not itself a substance, but a principle that cannot exist on its own, but only in tandem with a material factor together with which it constitutes a substance. A form-matter composite is not constructed from pre-existing 'building blocks.'
b) The souls of humans are not substances along Platonic-Augustinian-Cartesian lines, but forms of the bodies whose souls they are.
c) The souls of humans are subsistent forms that can exist on their own after the deaths of the bodies whose forms they are.
Can someone explain how all three propositions could be true? On the face of it, the first two, taken together, entail the negation of the third. What we have here is an inconsistent triad. Although collectively inconsistent, the members of the triad are individually plausible. Why plausible?
(a) is plausible given that (i) there is such a thing as ontological analysis and (ii) its style is hylomorphic. The authority of philosophus, The Philosopher, as Aquinas calls him, stands behind (a). The authority of Aristotle may also be invoked in support of (b) if the soul (anima, psyche) of a living thing is its life-principle. (The soul animates the material body of an animal, making it alive.) As for (c), it is a Christian commitment of the doctor angelicus that he cannot abandon.
The three propositions are collectively inconsistent but individually plausible. We've got ourselves a problem. Something has to give.
It has been said that Aquinas is a Platonist in heaven, but an Aristotelian on earth. These super- and sub-lunary tendencies comport none too well. One solution is to drop the Aristotelianism with its combined commitment to (i) hylomorphic ontological analysis and (ii) soul-as-life-principle and go the Platonic-Augustinian-Cartesian substance-dualist route. Richard Swinburne exemplifies this approach in Are We Bodies or Souls? (Oxford UP, 2019) A second solution would be to drop the Christian commitment to the immortality of the soul and embrace a form of materialism about the human person. A third solution would be to somehow uphold Christianity while accepting materialism about the human person. (See my Could a Classical Theist be a Physicalist?)
What other solutions are there?
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