The nature of the mind is best described by hylomorphism as opposed to idealism, substance dualism, and materialism. I will argue this for three reasons: first, hylomorphism solves the interaction and pairing problems of substance dualism; second, it solves the qualia and intentionality problems of materialism; and third, it offers a solution to the problem of what matter is, as raised by idealism. After arguing these points, I will then respond to some contemporary objections to hylomorphism.
Philosophical debates about the nature of the mind seek to understand what the mind is and whether it is material, immaterial, some combination of these two, or something else entirely like neutral monism. It’s an important discussion because it affects other important questions like whether or not there is an afterlife, whether machines could have minds, can studying the brain tell us everything about the mind, and others.
Hylomorphism, as argued by Aristotle and St. Thomas Aquinas, argues that a substance is composed of both form and matter. The substantial form acts upon or actualises matter to be what it is. (Aquinas, 1999b, para. 149-53; Aristotle, 1930, 193a28-31) These give rise to two intrinsic principles of the substance’s existence, being its formal and material causes. The two extrinsic principles are its efficient and final causes, or what brings a substance into being, and the purpose for which a substance was brought into being, respectively. Hylomorphism is distinct from substance dualism in that the immaterial (form) and material (matter) parts are not each distinct substances that only share an efficient causality. Hylomorphism is also distinct from materialism in that it doesn’t believe that substances can be reducible to just their physical or material components. Finally, hylomorphism is also distinct from idealism in that it doesn’t believe that ideas are uninstantiated in a material way. Instead, hylomorphism requires that a substance be understood by the four causes being the material, formal, efficient and final.
In hylomorphism, the soul is the substantial form of a living being. (Aquinas, 1999a, para. 235; Aristotle, 1987, 412b10-13a10) In Thomistic hylomorphism, the rational soul is unique in that it has an essential operation, i.e the intellect, which operates apart from the body. Aquinas argues that since the intellect is subsistent and operates incorporeally, then the human soul must also be incorporeal and subsistent. (ST I, Q. 75, A. 2) This makes Thomistic hylomorphism a kind of dualism since a human soul could survive the death of its body and still have a limited subsistent existence. (ST I, Q. 75, A. 6)  However, hylomorphism can also be considered a kind of monism, since it argues that every being is only one composite substance.
I believe that Thomistic hylomorphism solves the major problems of substance dualism, namely the interaction problem and the pairing problem. The interaction problem of substance dualism argues that if a material substance and an immaterial substance shared no properties in common, then there would be no mode in which they could interact. However, if there was no mode in which two substances could interact then they cannot interact. Therefore, material and immaterial substances cannot interact. While there are versions of substance dualism that deny that there is a causal interaction between a mental substance and a physical substance, like parallelism, they end up with the problem of being unintuitive since we understand that our mental processes are causing our body to act or function in different ways.
Hylomorphism solves this interaction problem because it doesn’t view causation as something purely mechanistic but argues that the actualisation of potential is the basis for change. For Thomistic hylomorphism, everything that exists is already an actualisation of some potential and so the mind actualises potential in the body as a formal cause to matter (top-down causation) and the body actualises potential in the mind as a material cause to form (bottom-up causation). (ST I, Q. 84, A. 6; I-II, Q. 17, A. 4) Since substance dualism treats all causation as either efficient or material, it is unable to account for the interaction between mind and body.
The second problem for substance dualism is the pairing problem. The pairing problem asks how an immaterial substance that lacks a spatial location could become attached to or associated with a particular material body. Since these two substances lack any sort of modal interaction, it seems as though there is no reason for why an immaterial substance would remain attached to or associated with some particular body. Hylomorphism also provides a solution to this problem. Since the soul is the substantial form of a particular body, then a body is contingent upon its soul for it to be what it is. The body and soul are intrinsically linked by their formal and material causes within a single composite.
I also believe that hylomorphism solves the major problems of materialism, namely the problem of qualia, and the intentionality problem. The problem of qualia, or the hard problem of consciousness states that while the mind/brain performs many functions, there is also an accompanying experience of that function. While functions can be explained by a physical process, it seems that what it is like to have an experience is not susceptible to a physical explanation. Therefore, there is something about consciousness that can’t be reducible to something physical and thus an element of the mind is nonphysical.
Hylomorphism is not committed to a purely material world and does not suffer the problem of having to explain non-physical qualia in materialist terms. Since forms are immaterial, qualia can be explained by some interaction between the body and the soul. Jaworski (2018, p. 1129), gives an explanation of how qualia can arise through the perception of an object in a hylomorphic framework by appealing to the unified event of the coordination of subsystems, and the interaction between subsystems, in the perception of an object by the immaterial form. The unified event of these causal interactions is produced by a supervenient coordination by the soul, it is this formal (relating to the form or soul) coordination that generates an experience or qualia. While this specific theory will be revised and refined by future philosophers, or perhaps even rejected for some better theory, it seems as though hylomorphism provides a sufficient solution to the problem of qualia by positing immaterial forms.
I also believe hylomorphism solves the intentionality problem for materialism. The problem of intentionality is that mental states are directed towards or contain events or objects beyond themselves. However, material entities never have content which is beyond themselves since matter is always contained or enclosed to itself. Therefore, mental states cannot be material. Another version of the argument is about intentional inexistence, it states that some objects of intentional states cannot be reducible to a physical existence outside of the mind. An example is that if I desired a wife, the ‘wife’ in this case does not need to refer to any specific woman who I would like to be my wife. Whereas if I said that a car accident involved a wife, then it would refer to a specific woman. The former use of the term wife seems to have no existence outside of the mind and thus cannot be reducible to something physical. However, if something exists in the mind, but doesn’t exist physically, then the mind has non-physical content and thus must also be non-physical itself.
Hylomorphism easily responds to this problem of intentionality by arguing for the existence of forms. Hylomorphism is generally committed to naïve or direct realism which allows for non-physical objects of intentional states to exist in the mind. Since these objects of intentional existence would be universal forms for Thomistic hylomorphism, the non-specific wife would just be the universal of wife existing in the intellect with the appetitive power desiring this universal as a good. This universal of wife wouldn’t need to refer to any specific woman or wife as the intellect knows universals, while the sense knows singulars. (ST I, Q. 86, A.1) However, even if hylomorphism wasn’t committed to naïve realism, the existence of the mind as partially immaterial allows the possibility of non-physical objects or events to exist in the mind.
I also believe that hylomorphism presents a good reply to difficulties raised by Idealism such as the question of what matter is. Berkeley defines matter as, ‘an inert, senseless, extended, solid, figured, moveable, substance, existing without the mind’. (Berkeley, 1996, p. 53) He goes on to say that no unperceived substance can truly be said to exist and since matter is imperceivable by his definition, it must also not be said to exist. However, hylomorphism doesn’t take matter itself to be some substance, but only matter with a substantial form. In this regard, hylomorphism would agree with Berkeley’s assessment of mind-independent material substance not existing, but not because matter doesn’t exist, but because matter requires a form or ‘idea’ and so cannot be independent or cannot be a substance by itself.
Aquinas would agree with Berkeley when he says, ‘for matter in itself can neither exist, nor be known.’ (ST I, Q. 3, A. 3, ad 3) Hylomorphism argues that change requires both act and potency, and that matter is potency that must be actualised by form. Thomistic hylomorphism argues that there is prime matter, and secondary matter. Prime matter is matter that lacks a substantial form, while secondary matter is matter that undergoes accidental change. This means that secondary matter is matter that already has a substantial form but undergoes accidental change like wood (secondary matter) that is turned into a chair. However, prime matter is the potential matter that the form of wood instantiates into actual wood. In the hylomorphic view, matter is necessary for change in objects. This conception of matter is not discussed by Berkeley, but I find it doesn’t end up with the same problems that his conception of matter has of being undefinable while also being perceivable. It also gives a significant reason for why someone should believe in the existence of matter, since we wouldn’t be able to account for change without it.
While hylomorphism offers a compelling solution to many contemporary problems in the philosophy of the mind, I also want to look at some contemporary objections to the theory. The first objection is that since hylomorphism argues that, ‘when certain material objects come to be structured such that those objects come to compose some numerically distinct composite whole, those same material objects continue to exist upon being structured in the relevant way’, this will lead to causal overdetermination. (Skrzypek, 2017, p. 382-84) To illustrate, if I built a table out of pieces of wood, then the table is numerically distinct from the individual pieces of wood, but the pieces of wood still persist even after being used to create the table. However, if I was to throw the table through a window and the window shattered, then the hylomorphic point of view would seem to be committed to saying that at least two things caused the window to shatter, the table and the pieces of wood. However, only one of these objects is sufficient to shatter the window and so there is an overdetermination by saying that both of the objects caused it.
My response to this objection is that it doesn’t take into account the distinction between substantive and accidental forms. The pieces of wood are substantive forms of wood matter, while the table is an accidental form of the pieces of wood. Anything with an accidental form is not strictly a substance according to Thomistic hylomorphism. While the pieces of the table are placed together in such a way as to create the accidental form of a table in the mind of the individual, and thus be numerically distinct but only in a mind-dependent way, the accidental form is not present in the table like a substantial form would be in the piece of wood. Therefore, we can avoid overdetermination by saying that properly speaking, only the pieces of wood are breaking the window.
A second objection centres on Thomistic hylomorphism's belief that the soul is the form of the body. (ST I, Q. 75, A. 5) Since, it appears that the body is itself already a substance, Aquinas would seemingly be saying that the soul is the form of a substance. Yet the form of a substance would either not be anything, or it refers to some abstract concept which would just describe the substance, but not have any content in itself. Therefore, Thomistic hylomorphism cannot account for an immaterial mind as being part of the soul as the soul has no content.
This objection is wrong in believing that Aquinas believes that the body is already a substance without the soul. Aquinas writes, ‘Of one thing there is but one substantial being. But the substantial form gives substantial being. Therefore of one thing there is but one substantial form. But the soul is the substantial form of man. Therefore it is impossible for there to be in man another substantial form besides the intellectual soul.’ (ST I, Q. 76, A. 4)
In conclusion, I believe that hylomorphism provides the most compelling case for understanding the nature of the mind. It provides sufficient solutions to the problems within substance dualism and materialism, and it responds to one of the main concerns about matter raised by idealism.
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———, Commentary on Aristotle's Physics, trans. by Richard J. Blackwell, Richard J. Spath and W. Edmund Thirlkel, (Notre Dame, Indiana: Dumb Ox, 1999b).
———, Summa Theolgiae, ed. by John Mortensen and Enrique Alarcón, trans. by Laurence Shapcote, (Lander, Wyoming: Aquinas Institute for the Study of Sacred Doctrine, 2012).
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———, De Anima (On the Soul), trans. by Hugh Lawson-Tancred, (London: Penguin, 1987).
Berkeley, George, Principles of Human Knowledge and Three Dialogues, (New York: Oxford World Classics, 1996).
Chalmers, David J., 'Facing up to the problem of consciousness', Journal of consciousness studies (1995), 2, pp. 200-19.
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———, Scholastic metaphysics: a contemporary introduction, (Heusenstamm, Germany: Editiones Scholasticae, 2014).
Jaworski, William, 'Hylomorphism and the Construct of Consciousness', Topoi (2018), 39, pp. 1125-39 <10.1007/s11245-018-9610-0>.
Skrzypek, Jeremy, 'Three Concerns for Structural Hylomorphism', Analytic philosophy (2017), 58, pp. 360-408 <10.1111/phib.12104>.
Smart, John Jamieson Carswell, 'materialism' (2022), Encyclopedia Britannica, Accessed 14 October https://www.britannica.com/topic/materialism-philosophy.
Van Inwagen, Peter, and Dean W. Zimmerman, Persons human and divine, (Oxford: Clarendon Press, 2007).
 Edward Feser, Scholastic metaphysics: a contemporary introduction, (Heusenstamm, Germany: Editiones Scholasticae, 2014) p. 160.  Ibid, p. 88.  Edward Feser, Philosophy of Mind, (London: Oneworld, 2005) pp. 221-22.  Ibid, p. 225.  Francis Bacon, The Advancement of Learning, ed. by Henry Morley, (London: Cassell, 1893) Bk. 2, Ch. VII, 3-5.  Amy Kind, Philosophy of Mind, (New York: Routledge, 2020) pp. 38-39.  David J. Chalmers, 'Facing up to the problem of consciousness', Journal of consciousness studies (1995), 2, pp. 202-04.  John Jamieson Carswell Smart, 'materialism' (2022), Encyclopedia Britannica, Accessed 14 October https://www.britannica.com/topic/materialism-philosophy.  Daniel Clement Dennett, Content and consciousness, (London: Routledge & Kegan Paul, 1986), p. 15.  Edward Feser, Scholastic metaphysics: a contemporary introduction, (Heusenstamm, Germany: Editiones Scholasticae, 2014) p. 171.  This argument is from Peter Van Inwagen and Dean W. Zimmerman, Persons human and divine, (Oxford: Clarendon Press, 2007), pp. 204-05.