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Thomist Anthropology and the Problem of Causal Interface

Published online by Cambridge University Press:  12 February 2025

Marcus Shane Otte Open the ORCID record for Marcus Shane Otte [Opens in a new window]
Marcus Shane Otte*
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Philosophy Department, University of Dallas, Irving, TX, USA
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Abstract

Aquinas’ anthropology is commonly believed to prevent the mind–body problem by treating the human being as one substance, and the soul as a formal cause. Thomists’ descriptions of Aquinas’ anthropology tend to understate or even omit its more dualistic elements, e.g., that the soul is an agent cause that moves the body, and that acts through the mediation of the ‘corporeal spirits’. More importantly, these descriptions overlook that Aquinas himself recognizes a problem of mental causality and even argues for some solutions to it. This paper aims to show that there is such a problem within Aquinas’ conceptual frame, and that contemporary Thomist anthropologies are also vulnerable to it.

Keywords

Thomas AquinasThomismphilosophical anthropologymind–body problemmental causality
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New Blackfriars , First View , pp. 1 - 13
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© The Author(s), 2025. Published by Cambridge University Press on behalf of Provincial Council of the English Province of the Order of Preachers.

1. Introduction

I am claiming there is a mind–body problem that Aquinas’ account of the soul engenders, and that Thomists generally fail to acknowledge it. Before proceeding, this claim should be disambiguated, as there are many kinds of mind–body problems, e.g., there is the ‘hard problem’ of consciousness, which is a question about how a material organism could have subjective experience. There is the ‘problem of mental causation’ about how mental states can affect the body. And there is the broader ‘interaction problem’ about the mind’s causality on the body, and the body’s on the mind. Of course, the formulation of these problems (and whether they are problems at all) depends upon one’s philosophical framework, e.g., how one parses the difference between the mental and the physical.

Since Thomists have non-modern concepts for describing the body and soul, and the material and immaterial, one would expect they do not face the same mind–body problems as their more mainstream contemporaries. Such an expectation is generally vindicated. With regard to the hard problem of consciousness, it has been noted that – although there are medieval versions of almost any contemporary philosophical problem – there is no medieval anticipation of this problem,Footnote 1 and therefore no medieval solution to it.Footnote 2 A similar remark can be made on the interaction problem. There is no Cartesian mind–body interaction for Aquinas, because the body is not understood to be a substance that could act from a power that is not also a power of the soul. One might speak of the body’s action on the soul, but considered more carefully, these are always acts of the soul–body composite upon itself.

But, what about the problem of mental causation? The contemporary problem of mental causation can be further subdivided into different species of problems, as materialists have rejected the conjunction of dualism and mental causation on a few different grounds: it is alleged to require the violation of scientific laws (in particular, conservation laws); it entails overdetermination, if the physical world is causally complete; it exacerbates the problem of other minds; etc. The classic and most enduring species of the problem is ‘the mysteriousness objection’, however (sometimes called ‘the causal nexus problem’). Alin C. Cuco and J. Brian Pitts say of this problem, that it, ‘involves the intuition that there does not seem to be any causal interface between nonphysical and physical entities that would allow the non-physical entities to interact with the physical world’.Footnote 3 Such an interface is supposed to be necessary, if the relation between cause and effect is to be intelligible. Thus, mental causation as dualists construe it is accused of unintelligibility.Footnote 4 This line of argument has a long history and has been given in many different versions. Princess Elizabeth of Bohemia puts this question to Descartes: ‘Given that the soul of a human being is only a thinking substance, how can it affect the bodily spirits, in order to bring about voluntary actions?’Footnote 5

Elizabeth’s question is motivated by the physics of the time, in which causality is a matter of pushing, requiring contact. Since the Cartesian mind is bereft of extension, such contact is impossible.Footnote 6 Thus, there is no way for mind and body to interface. That is to say, there is no appropriate link between them to explain a causal relation. Given its dependence on early modern physics, it is tempting to dismiss Elizabeth’s argument. But her broader point – that it seems there can be no causal interface between mind and body – arguably remains potent.Footnote 7 For the remainder of this paper, the question I refer to with the term ‘mind–body problem’ is the mysteriousness objection to mental causation. This objection is typically not specified beyond merely affirming that a causal relation between an immaterial mind and corporeal body is prima facie problematic, owing to the differences in their natures and properties. When the objection is formalized, its specification depends on the philosophy of nature that underlies it – for example, Jaegwon Kim has argued there can be no causal interface with an immaterial mind, because causal relations presuppose spatial relations.Footnote 8

There are two kinds of remedies prescribed for the mind–body problem: preventions and treatments, as John Peterson has put it.Footnote 9 Reductive or eliminative materialism prevents the problem from arising at all. So does idealism. And so does non-interactionist dualism, e.g., occasionalism or the pre-established harmony thesis. It is also possible to prevent the problem by reasoning along Humean lines: no interface between mind and body is needed, because causation is not intelligible. Causes and effects are loosely separate and are only identifiable thanks to constant conjunction. The Humean account of causation ameliorates the mind–body problem just by casting a skeptical eye on causal knowledge in general. Alternatively, one might preempt the mind–body problem not by taking a skeptical stance toward causal knowledge, but rather by regarding causality as primitive. Of course, these answers are not available to anyone who rejects such skepticism about causation, or construes it as non-primitive, as Thomists characteristically do.Footnote 10

Treatments are usually subtler. Possible treatments for the mind–body problem can be divided into two broad categories: those that try to establish a causal nexus and to thereby rebut the problem, and those that undermine the problem by showing that a given version of it has not demonstratively rebutted interactive dualism. Dualists have attempted to answer the mind–body problem primarily by undermining it.Footnote 11 In general, however, Thomists have shown little enthusiasm for such attempts, and instead have tended to regard them as futile. Thomists typically emphasize that there is no problem of ‘mind–body interaction’ within their view, precisely because the human composite is one substance, rather than two substances in interaction.

2. Thomist responses to the mind–body problem

Hylomorphism is presented by its proponents as, in part, a preventative measure against the mind–body problem. Indeed, this is supposed to be one of the chief merits of the theory. More specifically, the ordinary Thomist response to the mind–body problem is to argue that, if we understand human beings from within the same hylomorphism that Aristotle uses to account for other natural substances, the mind–body problem does not arise.Footnote 12 Formal causality, in particular, is supposed to be the key missing ingredient in modern metaphysics, and its absence from metaphysics the single most important factor in the apparent plausibility of the mind–body problem. Robert PasnauFootnote 13 finds it, ‘curious’ that the mind–body problem is still regarded as a problem, since it is a ‘historical artifact’ that presupposes early-modern convictions about the nature of corporeal objects and immaterial minds.Footnote 14 The problem does not crop up at all, if we regard the mind as just another natural power, so that, ‘our intellectual powers are just forms – powers of the soul – that can act in nature just as other forms, accidental and substantial, act in nature’.Footnote 15 Edward Feser writes in similarly strong terms:

Another consequence of the hylomorphic view is, arguably, that there is no mystery about how soul and body get into causal contact with one another, for the soul–body relationship is just one instance of a more general relationship existing everywhere in the natural world, namely, the relation between forms … and the matter they organize. If this general relationship is not particularly mysterious, neither is the specific case of the relationship between soul and body …. When it is allowed that there are other irreducible modes of explanation – in particular, explanation in terms of formal causation – the interaction problem disappears.Footnote 16

D.Q. McInerny, in his textbook The Philosophy of Nature, writes:

The mind–[body] problem is fictitious, that is, it is a non-problem, and that is because it is based on a totally erroneous understanding of the human person. If one accepts Descartes’ contention that the human person is somehow the composite of two distinct substances, one material, the other immaterial, then one has created for oneself the problem of explaining how mind and body communicate with one another, for they represent two radically different modes of being, material and immaterial. This pseudo-problem vanishes as soon as one recognizes the true nature of the human person, as a single substance for whom the relation between soul and body, the material and the spiritual, is essential, not accidental.Footnote 17

Gerard M. Verschuuren also asserts of hylomorphism:

This makes the interaction problem of substance dualism disappear, because there is no soul to be ‘in’ a body …. In the Cartesian view, a pilot can be without a ship, and a ship can be without a pilot, but in the Thomistic view, there is no body without a soul (unless it is a corpse), and there is no soul without a body (except temporarily, as we will see below).Footnote 18

In Aquinas, Eleonore Stump likewise writes that St Thomas’ anthropology prevents the mind–body problem.Footnote 19 This claim, however commonly it is argued, seems greatly exaggerated. Stump’s own writing can be brought forward to show just how implausible it is. On the one hand, Stump distinguishes Aquinas’ dualism from that of Descartes largely on the grounds that, for Aquinas, ‘there is no efficient causal interaction between the soul and the matter it informs’.Footnote 20 This turns out not to be such a great difference as first appears, however, because the ‘matter’ Stump is referring to is prime matter. Elsewhere, Stump writes that the will is an efficient cause of bodily motion, and the intellect acts efficiently on phantasms (which are embodied in brain states).Footnote 21 In other words, the soul is not an efficient cause toward prime matter but only toward the body, i.e., formed matter.Footnote 22 But if Aquinas views the soul as a subsistent particular that efficiently causes some bodily change, then anyone who objects to Cartesian interaction is likely to object to Thomism as well.

Gyula Klima likewise argues that Descartes’ mind–body problem does not afflict Thomism at all. Because soul and body are one substance, related as form and matter, rather than two complete substances, there cannot be any interaction problem. It is the unified organism that acts. Thus, on the basis of the substantial unity of the human being, Klima writes that, ‘there is no greater metaphysical mystery in the workings of the soul than there is in the workings of any complex natural phenomenon’.Footnote 23 Klima, however, does acknowledge that Thomas’ psychology faces a mind–body problem of some kind: ‘the question of interaction on the “interface” between the soul-informed (since living) brain, and the allegedly immaterial intellect’.Footnote 24 Even with this concession, too much is made of the differences between the two problems. This is so, in two respects.

First, for both Thomism and Cartesianism, the problem is one of causal interface between the immaterial and the material. As will be shown later in this paper, Aquinas regards the differences in nature between immaterial and material things as a problem for causal interface, but he does not think there is anything additionally problematic about a separate substance (i.e., an angel) moving a material body. It seems that Thomas’ objection to Cartesianism, if he were introduced to it, could not be that it renders the activity of an immaterial substance on a body to be especially dubious or puzzling.

Second, for materialism, the Cartesian viewpoint is objectionable because of the problem of causal interface; but by Klima’s admission, this is a difficulty for Thomism as well. So, we cannot put hylomorphism forward as a view that successfully evades the central concern of the Cartesian mind–body problem, since the very issue that afflicts Cartesianism (causal interface between the material and the immaterial) afflicts Thomism as well. While materialists do find it problematic that Descartes divides the human being into two substances and identifies himself with his mind, this is distinct from the problem of causal interface. There are few, if any, materialists who have regarded the division of human beings into two substances (popularly interpreted as each having a complete nature) as an aggravating factor in the causal interface problem.

An additional weakness of Thomist replies to the mind–body problem in general is that they tend to treat the soul–body relation as merely another instance of the form–matter relation, and the soul’s rational powers as mere instances of natural powers belonging to a substance. But the human soul and its faculties depart from the usual rules one might expect of forms, substances, and powers in Aquinas’ account of them. Only the human being is a composite substance with a form that is subsistent. Only the human being is a material substance with some powers that completely transcend its matter.Footnote 25

For these reasons, there is a problem of mental causation that is present in Aquinas. By placing so much emphasis on those respects in which Aquinas differs from Descartes, Thomists have created a distorted portrait of his psychology. Consequently, some of the difficulties faced by Thomist psychology have tended to be obscured, along with the resources Aquinas provides that might address these challenges. Hence, it goes unremarked that Aquinas recognizes a mind–body problem, and with very few exceptions, Thomists do not hold their view faces this problem.Footnote 26 These points will be explicated in the following sections.

The main problem with the responses above that I wish to address is that they do not really respond to the mind–body problem, as materialists formulate it. Though the human soul is not a substance in the strictest sense of having a complete nature (i.e., it is not able, on its own, to do everything that is natural for human beings to do), nevertheless, the human soul is subsistent on Thomas’ account of it.Footnote 27 And thus, it is a substance precisely in the sense that matters for the mind–body problem, and it is rightly called a ‘substance’ in the normal sense of the term as it is used in contemporary philosophy, to refer to a subject of properties.

If it is mysterious for an immaterial, subsistent thing to affect a corporeal body, then Aquinas’ view on the soul and its operations toward the body should be regarded as mysterious. The oneness of the human substance does not dissolve this mystery. Even if materialists can be persuaded to accept formal causality, together with the unity of the human person, they still must be persuaded that what is immaterial may bring about changes in corporeal bodies, as this is a distinct claim. There are challenges posed both by the dualism of the human soul and prime matter, and that of the human soul and body. The former concerns the soul’s formal causality, and the latter its efficient causality. It is the latter where a difficulty lurks that is like the Cartesian mind–body problem. Contrary to the commonplace portrayal of his psychology, Aquinas himself evidently held that there is such a problem, if by ‘problem’ we mean not an insolvable conundrum, but merely a question that may be considered as raising a difficulty.Footnote 28

3. Thomas Aquinas on the mind–body problem

First, it should be noted that Aquinas never objects to the Platonic view of the soul by posing an interaction problem. Rather, he objects because of the threat it poses to human unity, as it makes the union between body and soul accidental.Footnote 29 There are a number of incongruities that arise from making the body so extrinsic to the soul. Aquinas notes that the essential unity of the human being is evidenced by our acts of sensation: the acts of one’s senses are acts of the living body, and essentially consist in changes to parts of the living (i.e., ensouled) body, moved by the objects of sensation. But, if Platonism is the case, then the soul alone is the subject of sensation.Footnote 30

Further, Aquinas thinks, if the soul is only a mover and not a form, then it has the same relation toward the body that an angel may have toward a corporeal body: it would be a source of movement, but not a source of being.Footnote 31 In that case, the body does not owe its being to the soul. Since, for an organism, being is just the same as living, it would follow that the organism does not owe its life to the soul. It would even follow that ‘… death, which consists in the separation of soul and body, will not be the corruption of the animal. And this is manifestly false’.Footnote 32 Aquinas mentions a few other objections – that, if Plato is right, then it would follow that the union of soul and body is unnatural and harmful,Footnote 33 and that brute animals have subsistent souls that survive death.Footnote 34 So, if Aquinas does believe there is a mind–body problem, it does not seem that he regards Plato’s view as uniquely afflicted by it, as he does not object to Platonism on this ground.

Nevertheless, he does think there is a mind–body problem, and that his own metaphysics does not simply prevent it. This appears most plainly in his discussions of angels, wherein he makes comparisons to the human soul. Though Aquinas takes it as well-evidenced that the celestial bodies are moved by angelsFootnote 35; and that terrestrial bodies are moved by separate substances, both angelic and demonicFootnote 36; how such causal relations are possible is a distinct question. In the Quodlibetals, Aquinas responds to the question whether angels can act upon corporeal bodies. He poses, as an objection, a problem about causal interface: ‘Action can only occur between things that have something in common. But angels have nothing in common with bodies here below, since there is no genus common to corruptible and incorruptible things, as it says in Book X of the Metaphysics’.Footnote 37

Nor can the problem be solved by positing an intermediary body between an angel and the body it affects, as ‘a bodily medium cannot receive a spiritual impression’.Footnote 38 There is no proportion between the actuality of an angel and the potentiality of a corporeal body that would permit an angel, whether by a direct command or by another kind of influence, to change the form of a body.Footnote 39 Nevertheless, Aquinas thinks, angels can change corporeal bodies with regard to place – in other words, by instigating locomotion – upon command.Footnote 40 The ground for this possibility is that angels and corporeal bodies are not altogether dissimilar,Footnote 41 although Aquinas is not clear in the Quodlibetals what the relevant similarity is.Footnote 42

In the Summa Theologiae, he elaborates that locomotion is most akin to the angelic nature with regard to its nobility. Angels are relatively unchanging; they are as like to Pure Act as a creature may naturally be. For a corporeal body to change by locomotion is, in itself, a mere extrinsic change, as a thing’s place is not intrinsic to it. So, insofar as a thing has the potential to move by locomotion, it has the potential to change only as compared to something extrinsic to it.Footnote 43 The potential to locomotion is, then, the least indicative of imperfection and incompletion on the part of a corporeal body. For this reason, Aquinas thinks, if there is proportion between actuality on the part of the angels, and potentiality on the part of bodies, it is with regard to locomotion. Whatever we may think of this solution, it is evident that Aquinas does regard the dissimilarity between an intellectual substance and a corporeal substance as a difficulty, if we wish to say that the former affects the latter.

It is in the very context of discussing angelic causation, however, that Aquinas makes an important comparison with the human soul. Following the above explanation, Aquinas adds in passing: ‘Thus, the philosophers, too, have held that the highest bodies are moved with respect to place by spiritual substances. And we ourselves see that the soul moves the body primarily and principally by local motion’.Footnote 44 Elsewhere, in discussing the human soul, Aquinas raises a problem of causal interface very similar to the one he raises in the case of angels and bodies. Posing the question whether the human soul is joined to the body through the mediation of another body, he says on behalf of proponents of the view, first that Augustine seems to concur with them,Footnote 45 and also that ‘things that are distant from one another are united only through a medium. But the intellective soul is distant from the body, both because it is incorporeal and because it is incorruptible’.Footnote 46 In the replies, Aquinas disambiguates two distinct questions: how is the soul united to the body as a form? And, how is it united to the body as a motor?

In answer to the former, Aquinas answers that there is no mediating body. But, with regard to the latter, the ‘subtle’ parts of the body mediate the soul’s action to ‘the grosser parts of the body’.Footnote 47 To reiterate: the soul’s status as form is an adequate explanation for the substantial unity of the human being, but not an adequate explanation for human action, insofar as it is an act of the soul-body composite. Today, the alleged necessity for subtle corporeal parts to mediate the soul’s causal activity is almost exclusively associated with Descartes, even though it is present in Aquinas as well. I do not point this out to endorse Aquinas’ view on the corporeal ‘spirits’, but only to emphasize that Aquinas’ anthropology is more dualistic than it is commonly portrayed. More important, this passage again implies that hylomorphism is an answer to the problem of unity, but not, by itself, an answer to the mind–body problem.

Aquinas also notes the problem of causally interfacing material and immaterial things in a discussion of the punishment of the damned. He takes the stance that the separated souls of the damned are punished by a kind of corporeal fire, so that the fires of hell are neither immaterial, nor images in the imagination of the damned (as Avicenna thought), nor metaphorical.Footnote 48 Aquinas points to a number of objections to his view, at least two of which are relevant to how he would have understood the mind–body problem. He writes:

Objection 3. Further, according to the Philosopher (De Gener. i) and Boethius (De Duab. Natur.) only those things that agree in matter are active and passive in relation to one another. But the soul and corporeal fire do not agree in matter, since there is no matter common to spiritual and corporeal things: wherefore they cannot be changed into one another, as Boethius says (ibid.). Therefore the separated soul does not suffer from a bodily fire.

Objection 7. Further, every bodily agent acts by contact. But a corporeal fire cannot be in contact with the soul, since contact is only between corporeal things whose bounds come together. Therefore the soul suffers not from that fire.Footnote 49

It is the lack of matter and the impossibility of contact that, according to Aquinas, raises problems for the action of corporeal fire upon immaterial souls or angels. Similar difficulties arise for the action of immaterial souls or angels upon corporeal bodies.Footnote 50 Thus, for the sake of argument, Aquinas raises the objection that the soul must be a body, or else it could not move the body, since there can only be contact between bodies.Footnote 51 Elsewhere, he raises the objection that the soul cannot be united to the body without an incorruptible body intervening, as soul and corruptible body are too distant in nature.Footnote 52 Each of these, of course, bear some resemblance to later versions of the mind–body problem. While current science and philosophy of nature do not require physical contact for force to be applied to a body, nevertheless, there are those who argue that spatial relations are necessary for all causal relations whatsoever.Footnote 53 The belief that the distance between material and immaterial natures poses a problem for their interaction is more widely endorsed, at least as a credible intuition.Footnote 54

There is one final collection of evidence to consider, to prove that Aquinas regards the mind–body problem as a real one: the various solutions he posits for it. Presenting these arguments in their fullness, or identifying their bedrock premises that might be reworked into persuasive arguments currently, is not my objective here. Again, I only wish to demonstrate that Aquinas is cognizant of a mind–body problem, and that he does not regard it as a mere pseudo-problem. If he does think of it as a pseudo-problem, then he would not offer the answers he does: for example, that angels and souls are able to directly instigate locomotion (but not other kinds of corporeal change), since locomotion is the kind of corporeal change that is least indicative of imperfection, and thus, most akin in nature to incorporeal things.Footnote 55

On the topic of the soul’s movement of the body, Aquinas says the soul exercises efficient causality by directly moving the body’s ‘subtle’ parts.Footnote 56 On this point, he cites Aristotle’s Movement of Animals, the tenth chapter of which is about the necessity of the corporeal spirits. It argues that ‘that which is to initiate movement’ must be such that it ‘contracts and expands without constraint’ and is thus well suited to push or pull.Footnote 57 Aquinas compares human beings to the cosmos: the first cosmic change is the locomotion of the celestial spheres, which is ‘circular and continuous’.Footnote 58 This is supposed to be the source of all other corporeal change. In human beings, the heart is analogous to the celestial spheres. Its natural movement is the first movement of the body, and the principle of all the rest, and its motion – cyclical pushing and pulling – is ‘most like the motion of the heavens’, i.e., most like circular motion.Footnote 59

This motion, which follows naturally from the soul enforming the body and especially the heart,Footnote 60 is a necessary condition for the motions of the ‘spirits and humors’,Footnote 61 which are the ‘highest and simplest’ parts of the body.Footnote 62 Whereas the heart is moved by the soul qua its form, the spirits are moved, and have their movement determined, by the soul qua their motor. It is owing to their nobility and simplicity that the corporeal spirits are specially apt to be moved by the incorporeal soul qua motor. Here, Aquinas’ reasoning seems to be the same as his explanation for angelic causality on bodies, on which he cites Dionysius: ‘“God’s wisdom joins the ends of the primary things to the beginnings of the secondary things.” From this it is clear that a lower nature is touched at its highest point by a higher nature’.Footnote 63 There are correlations between the active powers of the soul, and the potencies of the most perfect parts of the body. All of this is alluded to where Aquinas answers the objection that the soul must be united to the body through the mediation of a subtle body or bodies.Footnote 64 There is no need for such mediation with respect to the soul as form, as it inheres ‘directly in the matter’,Footnote 65 giving it existence. But there is, with respect to the soul as motor.Footnote 66

Finally, Aquinas understands the agency of the soul as a kind of instrumental causality. This is where hylomorphism actually does come into play for Aquinas, as he regards all living composite things, and not only human beings, as having powers that can act as principal causes relative to the lower parts and powers of the organism, which are instrumental.Footnote 67 To establish that the soul is the form of the body is also to establish that the body’s parts are instruments of the soul.Footnote 68 In the case of the human being, there are powers that belong to the soul alone, and not to the body. This is precisely why a mind–body problem arises in the description of human beings, but not of other organisms.

4. Conclusion

There is a lacuna in the current literature on Aquinas that this paper aims to identify: a dearth of research on Aquinas’ responses to the mind–body problem. Consequently, there are also no answers to the mind–body problem given by contemporary Thomists that are grounded on Aquinas’ own thinking about the issue. The medieval problem of the unity of the human person is entirely distinct from the modern mind–body problem, and it is a mistake to regard Aquinas’ solution to the former as a preventative solution to the latter. As evidenced here, Aquinas does offer some arguments intended to overcome the mysteriousness objection, and his anthropology includes some features that remain underexplored. These could be fruitful sources for Thomists to draw on, in their own discussions of the mind–body problem. Perhaps most promisingly: to vindicate Aquinas’ theory of instrumental causality, in its account of the necessary and sufficient conditions of such causality, and to do likewise for his theory of hylomorphism, and to show that hylomorphism entails instrumentality, would be to establish a causal interface between the higher powers of the human soul and its lower (embodied) powers.

References

1 Peter King, ‘Why Isn’t the Mind–Body Problem Medieval?’ in Forming the Mind: Essays on the Internal Senses and the Mind/Body Problem from Avicenna to the Medical Enlightenment, ed. by Henrik Lagerlund (Dordrecht, The Netherlands: Springer, 2007), p. 204.

2 Edward Feser, ‘Aquinas and the Problem of Consciousness’, in Consciousness and the Great Philosophers, ed. by Stephen Leach, James Tartaglia (London: Routledge, 2017), p. 54.

3 Alin C. Cuco and J. Brian Pitts, ‘How Dualists Should (Not) Respond to the Objection from Energy Conservation’, Mind & Matter, 17 (2019), 96.

4 John Searle, The Mystery of Consciousness (New York: NYREV, Inc., 1997), pp. xii–xiii.

5 René Descartes and Princess Elisabeth of Bohemia, ‘Correspondence between Descartes and Princess Elisabeth’, trans. by Jonathan Bennett, Early Modern Texts, 1, <https://www.earlymoderntexts.com/assets/pdfs/descartes1643_1.pdf> [accessed 2 January 2025].

6 Ibid.

7 See Robb, David, John Heil, and Sophie Gibb, ‘Mental causation’, The Stanford Encyclopedia of Philosophy (Spring 2023 Edition), ed. by Edward N. Zalta and Uri Nodelman, sec. 2.1., <https://plato.stanford.edu/archives/spr2023/entries/mental-causation>.

8 Jaegwon Kim, Physicalism or Something Near Enough (Princeton, NJ: Princeton University Press, 2008), pp. 70–92.

9 John Peterson, ‘Persons and the Problem of Interaction’, The Modern Schoolman, 62 (1985), 131.

10 Nevertheless, something like the Humean reply might be available to Thomists. We need not concur with the Humean view on events as ‘loose and separate’ to at least concede that how causation happens is, at bottom, not empirically evident, even in the case of purely physical phenomena. It is evident that material objects move one another, but it is not empirically evident why or how this is the case. Scientific explanations eventually bottom out at the most fundamental physical level. Only metaphysics can provide further explanation, for example, in terms of a powers ontology. But metaphysical explanations are also the most fundamental that might be available for mind–body interaction. So, the mysteriousness of mind–body interaction might not pose a special problem for dualism (see Madden, Mind, Matter & Nature (Washington, DC: Catholic University of America Press, 2013), pp. 64–65).

11 To give a few examples: Timothy O’Connor has suggested that souls might have psychological properties that structure their relations and make causal pairing possible, in a manner analogous to the way spatial properties are required for physical things to exercise causality (‘Causality, Mind, and Free Will’, Philosophical Perspectives, 14 (2000), 105–17, at 107). Richard Swinburne defends interactionist dualism from claims that the empirical sciences already have, or at least could, disprove mental causality. He undermines arguments based on the causal closure of physics, as well as those based on Libet experiments (Mind, Brain, & Free Will (Oxford: Oxford University Press, 2013), pp. 104–12, pp. 117–23). He also suggests that quantum mechanics might provide space in nature for the soul’s causal activity (and, at the very least, deprives materialists of any simple argument against immaterial mental causes that appeals to determinism) Ibid., pp. 112–7).

12 It is exceptionally rare for Thomists to hold that their view entails a mind–body problem appreciably similar to that of Cartesians. Marco Stango does so, in ‘Understanding Hylomorphic Dualism’, Proceedings of the American Catholic Philosophical Association, 91 (2017), 147. Raphael Mary Thomas Salzillo O.P. does not concede that the mind–body problem is a real problem, but he does point out that Aquinas’ anthropology, ‘does indeed contain the elements that proponents of the interaction problem find objectionable’ in ‘The Soul As a Part in Aquinas’ (PhD diss., University of Notre Dame, 2019), p. 194.

13 Robert Pasnau, ‘Philosophy of Mind and Human Nature’, in The Oxford Handbook of Aquinas, ed. by Brian Davies and Eleonore Stump (New York, NY: Oxford University Press 2012), p. 364. I cite Pasnau because – while it would be controversial to call him a Thomist – he is certainly a very ‘Aquinas-friendly’ philosopher, as Gareth B. Matthews calls him, in his review of Thomas Aquinas on Human Nature: A Philosophical Study of Summa Theologiae 1a 75–89, by Robert Pasnau, Notre Dame Philosophical Reviews, University of Notre Dame Department of Philosophy, July 2007, <https://ndpr.nd.edu/news/thomas–aquinas–on–human–nature–a–philosophical–study–of–summa–theologiae–1a–75–89/>.

14 Pasnau, ‘Philosophy of Mind and Human Nature’, p. 364.

15 Ibid.

16 Edward Feser, Philosophy of Mind: A Short Introduction (Oxford: Oneworld Publications, 2018), p. 223. Emphasis added. This book is an introductory work, it should be noted, but this passage is representative of Feser’s stance on this question. See Feser, Scholastic Metaphysics (Heusenstamm: editiones scholasticae, 2014), pp. 14–15; Aristotle’s Revenge: The Metaphysical Foundations of Physical and Biological Science (Neunkirchen-Seelschied: editiones scholasticae, 2019), p. 92.

17 D.Q. McInerny, The Philosophy of Nature, 3rd edition (Fraternity Publications, 2014), p. 423, n. 22. Emphasis added.

18 Gerard M. Verschuuren, Aquinas and Modern Science: A New Synthesis of Faith and Reason (Kettering, OH: Angelico Press, 2016), p. 191.

19 Eleonore Stump, Aquinas (New York: Routledge, 2005), p. 210.

20 Ibid.

21 Ibid., 264, 279. It should be noted that there is a difference between the activity of the will on the sensitive powers, and the activity of the active intellect on the phantasm. On one hand, it seems clear that the activity of the will must make a physical difference in the brain, especially when there is a consequent act ‘imperated by the will’, e.g., an emotion, imagining, local motion, or thought which the will has commanded (William Wallace, Elements of Philosophy: A Compendium for Philosophers and Theologians (Eugene, OR: Wipf and Stock Publishers, 1977), p. 152), allowing that the will’s command over the body’s limbs is ‘despotic’, whereas that over the emotions is ‘political’, as these naturally have their own tendencies that may resist the will (See Aquinas, Summa Theologiae, trans. by Alfred Freddoso (University of Notre Dame), <https://www3.nd.edu/~afreddos/summa-translation/TOC-part1.htm>, I-II q. 56 a. 4 ad. 3).

On the other hand, it is not clear that the intellect affects the brain – altering the brain-state that materially constitutes the phantasm – when it illumines the phantasm. Neither is it clear that the brain-state is not affected. Prior to abstraction, the intellect illumines the phantasm, so that ‘by the power of the active intellect the phantasms are rendered suitable for intelligible intentions to be abstracted from them’ (Summa Theologiae, trans. by Freddoso, I q. 85 a. 1 ad. 4).

22 Robert Pasnau, review of Aquinas, by Elenore Stump, Mind 114 no. 453 (Jan. 2005): p. 205, https://www.jstor.org/stable/3489000.

23 Gyula Klima, ‘Thomistic “Monism” vs. Cartesian “Dualism”’, Logical Analysis and History of Philosophy, 10 (2007), 108.

24 Gyula Klima, ‘Aquinas on the materiality of the human soul and the immateriality of the human intellect’, Philosophical Investigations, 32 (2009), p. 172.

25 This is not to say that these powers’ modes of operating during our earthly lives do not depend upon any organs – they certainly do. But, it is to say that the acts of the intellect and the acts of the will are not acts of any organ.

26 Marco Stango has explicitly affirmed there is a mind–body problem in Thomism and has even indicated that Aquinas was aware of it. See ‘Understanding Hylomorphic Dualism’, pp. 151–52.

27 Aquinas, Summa Theologiae I q. 75 a. 2.

28 William Jaworski’s, Structure and the Metaphysics of the Mind: How Hylomorphism Solves the Mind-Body Problem (Oxford, UK: Oxford University Press, 2016): is omitted here, just because it is not Thomistic. However, it also puts hylomorphism forward as a preventative solution. Jaworski notes that hylomorphism does not per se entail any particular view on a distinction between the mental and physical. Since the mind–body problem can only arise if such a distinction is posited, the problem need not occur for a hylomorphic view of the human being (pp. 175–177, 324–325).

I also wish to note here why I have not included David Cory’s ‘Thomas Aquinas on How the Soul Moves the Body’, in Robert Pasnau, ed., Oxford Studies in Medieval Philosophy vol. 8 (Oxford University Press, 2020), in the summary above. Cory’s article does not take an explicit stance on whether there is a mind–body problem in Aquinas, or what a solution might be to such a problem. Rather, the article is focused primarily upon the souls of plants and brute animals and how they are functionally different from the forms of inanimate beings: why inanimate beings cannot move themselves, and why their forms are not movers, whereas animate beings do move themselves, and their souls are movers. In short, Cory argues that the two roles of a soul – to be a form and to be a mover – are only conceptually distinct, and that the soul accomplishes both roles in the very same activity. A soul is a mover just in this sense: that it ‘gives both the whole and the parts of such an organic body their specific character’ (p. 176). Because it renders the body complex, and not homogenous, the parts of a body are able to act on its other parts. Cory acknowledges that this account is not completely adequate to the human case: human acts involve powers that are not embodied by the soul’s enforming activity (p. 180). But still, he does not make explicit any position on whether there is a mind–body problem about human acts, and he distinguishes the question his paper is focused upon from the question ‘of the interface of soul and body’ (p. 175).

29 Aquinas, Summa Contra Gentiles II, trans. by James F. Anderson (Notre Dame, IN: University of Notre Dame Press, 1975), cap. 57.3.

30 Ibid., cap 57.6–8.

31 H.D. Gardeil, Introduction to the Philosophy of St. Thomas Aquinas, Vol III: Psychology (Eugene, OR: Wipf & Stock, 2009), pp. 31–32.

32 Aquinas, Summa Contra Gentiles II, cap 57.11–12. See also Aquinas, Quaestiones Disputatae de Anima, a. 1.

33 Aquinas, Summa Contra Gentiles II cap 83.12–13; Aquinas, Quaestiones Disputatae de Anima, a. 2. ad. 14.

34 Aquinas, Summa Contra Gentiles II, cap 57.9.

35 Ibid,. II, cap 97.4; Summa Theologiae I q. 110 a. 1.

36 Aquinas, De Substantiis Separatis, cap. 2.11; cap 14.79.

37 Thomas Aquinas, Quodlibetal Questions, trans. by Turner Nevitt and Brian Davies (Oxford University Press, 2020), IX q. 4 a. 5 obj. 1.

38 Ibid., IX q. 4 a. 5 obj. 2.

39 Ibid., IX q. 4 a. 5.

40 Ibid., IX q. 4 a. 5 ad. 2.

41 Ibid., IX q. 4 a. 5 ad. 1.

42 He says only: ‘Angels do have something in common with bodies here below – namely the commonality between a mover and what it moves’ (Ibid.).

43 Aquinas, Summa Theologiae, trans. Freddoso, I q. 110 a. 3.

44 Ibid.

45 Aquinas, Summa Theologiae, I q. 76 a. 7 obj. 1.

46 Aquinas, Treatise on Human Nature: The Complete Text (Summa Theologiae I, Questions 75–102), trans. by Alfred Freddoso (South Bend, IN: St. Augustine’s Press, 2010), I q. 76 a. 7 obj. 3.

47 Ibid., I q. 76 a. 7 ad. 1.

48 Aquinas, Summa Theologiae, Supplement q. 70 a. 3; q. 97 a. 5.

49 Aquinas, Summa Theologiae, trans. by Fathers of the English Dominican Province (Christian Classics, 1981), Supplement q. 70 a. 3.

50 Except with this key difference: the direction of causality. The punishment of hellfire is more metaphysically problematic than the activity of angels or of souls upon bodies, because nothing corporeal has the natural power to affect what is incorporeal. Hence, Aquinas answers the objections posed above by appealing to supernatural causality. The corporeal fires of hell are used instrumentally by God, and thus, are able to achieve an end that they are not naturally capable of by themselves: to restrict the motion of a spiritual being, so that it is not free to exercise its power and make itself present elsewhere in space (Ibid., Supplement, q. 70).

51 Aquinas, Summa Theologiae, I q. 75 a. 1 obj. 3.

52 Ibid., I q. 76 a. 7 obj 3.

53 See Kim, Physicalism, or Something Near Enough, pp. 78–87.

54 It is worth asking why this is not also a problem for divine causality on material things – or at least, why divine causality on bodies has not attracted anywhere near the same degree of philosophic consternation. The brief answer is simply an appeal to divine omnipotence; but, fuller answers addressing the problem of divine causality from within a given metaphysical frame, are possible. There are at least two lines of explanation for God’s causality on bodies, which we can glean from Aquinas. First, the divine omnipotence is founded on the divine nature: Subsistent Being Itself. Aquinas holds that ‘each agent effects what is similar to itself’ (Summa Theologiae, trans. Freddoso, I q. 25 a. 3), so that there is a correlation between an active power and what is possible to it. But, the divine nature is pure esse, unrestricted by any limiting essence, and not confined to any genus. So, God has the power to produce any finite being whatsoever, as any such being is similar to God, qua being (Ibid.) Second, since God creates the whole of a composite creature, both its matter and form, insofar as these both participate in being (Ibid., I q. 44 a. 2), He is able to impress a form upon matter (Ibid., I q. 105 a. 1). Even among natural causes, what is able to impress a form is also able to cause the movement that is consequent on the form. So, God can directly move a body. Indeed, ‘It is a mistake to claim that God cannot effect by Himself all of the determinate effects that are brought about by any created cause’ (Ibid., trans. Freddoso, I q. 105 a. 2). It is God’s power as creator that entails He can also directly impress forms on creatures and can directly move them; spiritual creatures are only able to directly move a body locomotively, and thereby to indirectly bring about other changes (Ibid., I q. 105 a. 1 ad. 1).

55 Aquinas, Summa Theologiae I q. 110 a. 3.

56 Aquinas, Aquinas, Treatise on Human Nature, trans. Freddoso, I q. 76 a. 7 ad. 1.

57 Aristotle, Movement of Animals, X, in Jonathan Barnes, ed., The Complete Works of Aristotle, vol. I (Princeton, NJ: Princeton University Press, 1984).

58 Aquinas, De Motu Cordis, trans. by Gregory Froelich, St. Isidore e-book library, <https://isidore.co/aquinas/english/DeMotuCordis.htm> [accessed 2 January 2025].

59 Ibid.

60 Ibid.

61 Aquinas, Quaestiones Disputatae de Anima, trans. by John Patrick Rowan, HTML edition by Joseph Kenny, O.P., St. Isidore e-book library, a. 9 obj. 15 <https://isidore.co/aquinas/english/QDdeAnima.htm> [accessed 2 January 2025] See Aquinas, Quaestiones Disputatae de Anima a. 9 ad. 15.

62 Ibid., a. 9 obj. 15.

63 Aquinas, Summa Theologiae, trans. Freddoso, I q. 110 a. 3.

64 See Aquinas, Summa Theologiae, I q. 76 a. 7 obj. 1-3.

65 Aquinas, Treatise on Humane Nature, trans. by Freddoso, I q. 76 a. 4 ad. 3; ibid., I q. 76 a. 6 ad. 3.

66 Ibid., I q. 76 a. 7 ad. 1.

67 Aquinas, Commentary on De Anima, trans. by Kenelm Foster, O.P. and Sylvester Humphries O.P. (New Haven, CT: Aeterna Press, 2015), Book I, n. 19; Book II, n. 321-322, n. 332.

68 See Aquinas, Summa Theologiae, I q. 76 a. 5 ad. 3; III q. 8 a. 2. One might wonder, especially given the prominence of supervenience in philosophy of mind, whether these instrumental relations could be given a supervenience reading, rather than a causal one. Of course, this would mean that lower powers and bodily states supervene on higher powers and intellectual or volitional states, rather than vice versa (thus, materialism is avoided). The appeal of this proposal is that it would not require a causal interface. However, this proposal would not really fit Aquinas’ anthropology. A supervenience relation implies a co-occurring relation of determination: if phantasms supervene on intelligible species, then a sameness of intelligible species entails a sameness of phantasms. Likewise, if actions of the sentient appetite supervene on acts of the will, then a sameness in acts of the will entails a sameness in the motions of sentient appetite. But, in both of these propositions, the consequents are false. It is possible for two minds to bear the same type of intelligible species, from radically different phantasms; and it is possible for two free agents to make acts of will of the same type, while having very different motions of the sentient appetite.