By Paul Chutikorn
In the field of philosophy, and more particularly, the philosophy of the mind, one encounters what is commonly referred to as the mind-body problem. This concept came about even in pre-Aristotelian times, but became a major topic of emphasis concomitant to the philosophical method of French philosopher René Descartes. He introduces us to this problem based upon his unique view of substance, one which flows from his understanding of the self as a thinking thing. He believed that mental substances were necessarily distinct from material substances, and that the total essence of the human person was the mind.  Yet still, even Descartes realized that there was an intimate connection between the mind and the body, and that we needed an explanation for precisely what the nature of this connection is. However, Descartes fails to find this connection between the mind and the body and how they interact with each other. Moreover, the system employed by St. Thomas Aquinas provides a much more coherent understanding of the relationship between the two. Adopting Aquinas’ view of substance will provide a solution to the problem by avoiding altogether the position that man is made up of dual substances. Rather, Aquinas shows us that we can acknowledge a duality within substance itself, while maintaining its inherent substantial unity.
Descartes’ belief that the essence of the human person is the mind stemmed from his distinction between the mental substance (res cogitans) and the material or extended substance (res extensa). His conclusion was a natural consequence to his universal method of doubt, one which led to his concept that the mind as a thinking substance was the only thing that can be clearly and distinctly perceived. The perception of extended objects in space, then, were considered as loose and separate concepts. Descartes thought that the only indubitable truth was that of the existence of res cogitans since, in order for one to doubt, there must be someone who is doubting. Thus, it is certain that at least the person thinking is a thinking thing. This is, of course, what caused him to conclude with his well-known phrase, cogito ergo sum – I think, therefore I am.
According to Descartes, as one can see, the distinction between the two separate substances of mind and body is made because the mental substance operates independently from material substances. To demonstrate further, Descartes gives the example of a chiliagon (a thousand-sided polygon) which can be conceived in the mind as pure intellection but cannot be pictured in the imagination.  The reason for this is because with a triangle, we are able to both conceive of a three-sided polygon as an idea and look upon the triangle in the imagination by means of comparison to that which is corporeal. But this thousand-sided polygon cannot be imagined since the imagination turns to the body, and no such bodies exist, while intellection turns to the mind which is not limited to material bodies. The chiliagon is clearly distinct from any physical chiliagons being that it has not been perceived extraneous to the mind. The mind must, by necessity, be distinct from the body.
While others have attributed to Descartes a radical separation between the mind and body, he himself makes clear that there is a definite connection between the two as interacting, albeit distinct substances. He sees that there must be a unity present since when the body is hurt, we perceive pain, and likewise when the body needs food or drink, the mind has immediate knowledge of this. But how can we explain this interaction between two separate substances? Descartes claims that there is a small gland in the brain by which the soul (i.e. mind) moves the body to act. This gland has now been identified as the pineal gland which is a small endocrine gland at the base of the brain. While Descartes certainly approaches the mind-body relation as one that is mechanistic in nature, he does not see it as a radical case of being “lodged in a body as a pilot in a vessel” but rather, that the “mind and the body compose a certain unity.” However, this unity is never fully explained by Descartes, so the Cartesian concept of mind-body relations seems to leave more to be desired.
St. Thomas Aquinas has a very different view of substance, and therefore an entirely different view about the nature of the union between the body and the soul. For Aquinas, the human person is not a composite of two substances. Following Aristotle, he holds that the human person (as with any substantial being) must be one substance which is a composite of the metaphysical co-principles in material beings (e.g., prime matter and substantial form). Although, this is not to say that Aquinas would disagree with Descartes on all issues pertaining to the mind and body. On the contrary, he actually agrees with Descartes insofar as he holds that the intellect is distinct from the body. Aquinas explains that the intellect must necessarily be distinct from the body by virtue of its capacity of abstraction and understanding of universals. He says that if the intellect itself was a body, it could only understand particulars because the intelligible forms would be received as individuated. To use more Cartesian terms, if the intellect was extended, it would not be able to abstract since we could not understand things according to their forms. For instance, we would look at a tree and instead of being cognizant of its universal treeness, we would only see it as that particular tree without truly knowing of its nature or essence.
So, since Aquinas agrees that the mind is not the body, it would seem that he is a substance dualist after all. This is not the case in the strict sense, however, because Aquinas does not speak of the soul in reference to res cogitans as Descartes does. Alternatively, if Aquinas speaks of the mind, he is speaking in reference to the intellect which itself is a faculty of the soul. For Aquinas, the human person has an intellect, but when he speaks of the substantiality of a human person, he thinks in terms of matter and form, (viz., body and soul). The soul, for Aquinas, is the substantial form of any living thing. But the substantial form of a human person is different in quality than that of a plant or a non-rational animal in that it has both corporeal and incorporeal operations. Thus, the human soul is a substantial form of a substance that is corporeal insofar as a human being has a body, and incorporeal insofar as a human being has an intellect.
The point above is that, contra Descartes, Aquinas does not consider the human being as that which is composed of two separate substances, with the mind being the essence of the person. Rather, according to the principle agere sequitur esse (action follows being), he shows that the person is substantially both mind and body as a unity since its actions include both incorporeal and corporeal operations. He says that, “one cannot sense without the body, therefore the body must be some part of man.” Additionally, since one cannot understand according to the mode of human understanding without the intellect, the substantial form of the human person (which provides the faculty of the soul) must be some part of man as well. Therefore, man is both body and soul, according to Aquinas.
But what would Aquinas make of the concept of the chiliagon that Descartes brings up to show the separability of the intellectual powers and the imagination which is dependent upon extended bodies? First, Descartes makes the mistake of assuming that what he thinks about something is equivalent to the properties of the thing itself. Aquinas would likely bring up the fact that there is a difference between conceptual distinctions and real distinctions. One such example is that there is a conceptual distinction between Superman and Clark Kent, but this conceptual distinction does not mean that both Superman and Clark Kent are different people. The other example is that of a glass which contains water up to the halfway point. There are conceptual distinctions that can be made such as the glass being half full or half empty. However, these are conceptual distinctions of the mind, not real distinctions because they only exist in the mind. Outside of the mind, it is a glass that contains water to the halfway mark. Aquinas would say that the operation of the intellect, while it depends upon the reception of sense data in order to do its work, can certainly conceive of things that are not real.
It is also worth noting that Descartes’ attempt to impart unity upon the mind and body through the pineal gland seems unintelligible considering the fact that it would be metaphysically impossible for what he considers to be two separate substances to become, in some sense, one. It is especially incoherent to suppose that an immaterial substance unites with a material substance through something that is itself material. It is much more conceivable to posit that there is no central meeting point, but rather that the substance of the human person (while having distinct internal principles of body and soul) is one substantial being with two principles that operate in distinct ways.
It is essential to understand that none of this means our soul (as the principle of life or the substantial form of our matter) is an altogether distinct substance from the body. What it means is that the soul is a correlative principle that makes up the human person who is one substance in the hylomorphic compound of body and soul. It is this understanding that truly makes sense of the interaction between that which is corporeal and that which is incorporeal. It cannot be a part of the brain which connects the two; it is simply that the mind and the body are attributes of one substance which make up the whole of the human person. Resorting to the Cartesian approach of substance dualism makes it such that there is no feasible solution to the mind-body problem. Thus, an appeal to the Aristotelian-Thomistic view of substance merges into perfect harmony the truths that are contained in both the rationalist and the empiricist philosophical systems.
 René Descartes, Meditations on First Philosophy, VI, 2.
 René Descartes, Meditations on First Philosophy, VI, 3.
 René Descartes, Meditations on First Philosophy, VI, 13.
 René Descartes, The Passions of the Soul, I, 31. in Descartes Philosophical Writings, (trans. NK Smith. New York: Modern Library, 1958).
 Solomon, Robert C., Kathleen Marie. Higgins, and Clancy W. Martin. Introducing Philosophy: A Text with Integrated Readings. New York: Oxford University Press, 2016, 339.
 René Descartes, Meditations on First Philosophy, VI, 13.
 Thomas Aquinas, De Principiis Naturae, I, 3, 5., trans. R.A. Kocourek at the Dominican House of Studies Priory.
 Thomas Aquinas. Summa Contra Gentiles, II, 49, 4., trans. James F. Anderson, at the Dominican House of Studies Priory.
 Edward Feser, Aquinas on the Human Soul in The Blackwell Companion to Substance Dualism, First Edition. Edited by Jonathan J. Loose, Angus J. L. Menuge, and J. P. Moreland. 97.
 Thomas Aquinas, Summa Theologiae, I, q. 76, a. 1.