The immateriality and incorruptibility of the human soul, as well as the return of the soul to the body in the resurrection, are doctrines of the Catholic Church established by Scripture and Tradition as articles of faith. Nevertheless, these propositions can also be explained through the philosophy of St. Thomas Aquinas, and so by reason alone, the immortality of the soul and the fittingness of the resurrection of the body can be ascertained.
According to the metaphysical system of St. Thomas, a substance is defined as an identifiable unity “standing under” and perduring through the actions it may perform or the changes it may undergo. In this way, a substance is said to be subsistent, with its proper operations performed through itself (per se), and thus it “exists per se rather than through another.” A natural substance, complete in its nature as “a whole entity subsisting in itself and not as a part of any other being”, is composed of an act of existence, its finite and unique participation in the infinity of being, and an essence which limits the range or modes of being that can be expressed through its actions in an individual nature. Essence is further specified in the philosophy of St. Thomas by form, through which an individual essence participates and shares in a nature which it holds akin to other beings of the same species. A form is thus considered essential or substantial if it is subsistent, capable of existing and operating in its specific mode through its own power. For composite substances, including all those which constitute the material world, form is individuated by or “according to” matter, with form actualizing and expressing its particular nature, its unique participation in being, through the potencies of the material parts which it organizes and unifies into a single, identifiable substance, while matter fixes form to a concrete extension in space and time occupied by it alone.
A thing can be a substance in two ways: by subsisting through itself, or by subsisting through itself as a complete nature. St. Thomas thus distinguishes between the human soul as “the form of the body,” and the nature of the hypostatic human person as a union of the metaphysical co-principles of body (matter) and soul (form). The human soul can be considered subsistent because it is intellectual; although it subsumes other powers or operations, including sensation and vegetative growth, a soul is defined by its hierarchically highest power, that which is the most universal and least material and to which its lower powers are oriented as to their end. For humans, this defining operation is the intellect, which St. Thomas describes using the analogy of light as the ability to unveil the truth of reality and conform itself to it. Intelligibility or truth is thus revealed as a transcendental or universal property of being, with all existent things having an inherent order and form which make them capable of being understood by an intellect. In this way, the intellect, and thus the soul of which it is its highest function, is revealed to be both immaterial and incorruptible.
St. Thomas provides several proofs or arguments to corroborate the immateriality and incorruptibility of the human soul, based primarily on the intellect’s knowledge. First, in what is considered by him to be the “main reason”, the intellect can know a diversity of extant beings. As St. Thomas explains,
For it is clear that by means of the intellect man can have knowledge of all corporeal things. Now whatever knows certain things cannot have any of them in its own nature; because that which is in it naturally would impede the knowledge of anything else… Therefore, if the intellectual principle contained the nature of a body it would be unable to know all bodies.
Moreover, if the intellect were physical, or dependent upon a physical organ (like the brain), it would have only a specific proper object: “It is likewise impossible for it to understand by means of a bodily organ; since the determinate nature of that organ would impede knowledge of all bodies”. However, this is shown not to be the case, since the intellect has no limitation in what it can know, nor is it suppressed but rather is perfected by learning new things beyond its former knowledge: “For an intellect that understands the highest of intelligible objects is more able afterwards to understand those that are lower.”
The second proof explicates the process of the first by specifying that the intellect does not know the particular or individual things of sensory experience, but rather knows the universal forms of things by abstracting the intelligible species from impressions made on sense organs and stored in the imagination:
The intelligible species is to the intellect what the sensible image is to the sense. But the sensible image is not what is perceived, but rather that by which sense perceives. Therefore the intelligible species is not what is actually understood, but that by which the intellect understands.
As Dr Edward Feser explains, abstraction “strips away all particularizing or individualizing features of a phantasm so as to produce a truly universal concept or ‘intelligible species.’” Further, if the intellect were material, it would to some extent become that which it knew, and so whenever it knew a tree, it would become a tree or tree-like, as form existing in matter. This is obviously not the case; instead, “the intellect can, unlike material things, take on the form of other things… without losing its own form.” The intellect can also know abstractions and transcendentals not based in sense experience: “There are certain things we can form no mental images of – abstractions like law, love, and economics, the absence of a thing, and so forth – but the intellect can easily form concepts of them.” Thus, although the intellect ordinarily depends upon mental images or phantasms for its operations, as the objects “by which the intellect understands”, it is superior to the imagination, since images are only of singulars but concepts are of universals, contrary to “modern ‘computational’ accounts of the mind.”
St. Thomas also offers interior proofs for the immateriality of the soul. As shown by the ubiquitous human belief in an afterlife, ultimate truth and the moral significance of our actions, all humans desire immortality, even those who do not believe in its fulfillment. Since “the intellect apprehends absolutely, and for all time”, it naturally desires immortality. For St. Thomas, no natural desire can be futile and left dissatisfied; thus the soul must be destined for immortality, just as the appetites of hunger, thirst, etc. prove the existence of food, water, etc. Interior self-reflection is also given as a proof:
The human soul seems to be raised above the body because we can reflect on our actions and thinking, which is from above. St. Thomas thought that self-reflection shows that the human soul is immaterial, for he notes that when we think, we also think of our thinking, but no material organ can do this.
From this foundation, St. Thomas goes on to clarify that, although the human intellectual soul is immaterial and thus subsistent and incorruptible, it is not a complete natural substance by itself, since some of its powers require a body for their operations and so both soul and body are part of the definition of human nature: “Not every particular substance is a hypostasis or a person, but that which has the complete nature of its species”, and so “the persistence of the soul after death does not amount to the survival of the person; when John dies, his soul carries on, but he does not, at least not strictly speaking.” As Fr. Francis Selman explains, the person is the complete substance as subject, not only the mind which it uses, thus “by making the mind a power of the soul and holding that the soul is united to a human being as his or her form, St. Thomas shows that it is the whole person who thinks.” From this perspective, it could seem unnatural or pointless for the soul to be immortal, since on its own it would remain disembodied and impersonal for eternity with no hope of regaining its personhood and the restoration of its full hypostatic nature. However, as a Catholic St. Thomas affirmed the doctrine of the resurrection of the body, enabled by the Resurrection of Jesus Christ. While this is an article of faith, St. Thomas allowed his faith to illuminate his reason. Faith provides answers which surpass reason, yet it cannot be in conflict with reason; instead, it provides material which answers the questions of reason in a fuller and more definitive way.
In answer to the perplexity of the disembodied immortal human soul, the resurrection reveals that God will return souls to their restored bodies. In this way, the soul will not remain incomplete, with its bodily powers left inert and its personhood unrecoverable, but rather all people will again become complete substantial persons in the reunification of soul and body – both those who are saved and those who are damned. Although this cannot be proven by reason alone, St. Thomas shows that it is immanently fitting and suitable, since it is unnatural for a thing to persist in existence with an incomplete nature. As Fr. Selman explains,
St. Thomas thought that belief in the immortality of the soul leads to belief in the resurrection of the body. Since the human soul is by its nature the form of the body and it can be shown that it is immortal… but nothing contrary to nature can remain forever, the soul must be reunited with the body. ‘The immortality of the soul seems to demand the future resurrection of bodies.’
Another validation of the fittingness of the resurrection is in the fact that human souls are individuated according to matter, since the soul is the form of the body, and it does not exist prior to the body; disembodied, souls only remain individual by their “commensuration” to a body during life, with the soul developing its own unique history, identity and personality in the substantial union of soul and body. Even after death, the soul retains what Fr. W. Norris Clarke calls a “natural inclination” to its particular body, and so its individuality would be only reminiscent, in hindsight so to speak, without the resurrection. Fr. Selman elucidates further: “As the human soul does not perish with the body, because it is subsistent, so neither does its individuality, which it acquired by being in this one body.”
In conclusion, the metaphysical system of St. Thomas Aquinas, in light of his Catholic faith and Aristotelian philosophy, provides sufficient reasons for believing in the immateriality and incorruptibility of the human soul, as well as the fittingness and suitability of the bodily resurrection from the dead. While the resurrection can only be known by superrational faith, it can be shown to fulfill the questions of reason in a most perfect way and to satisfy our metaphysical desires with the fullness and propriety which alone can guarantee happiness, the satisfaction of all natural desires.
 W. Norris Clarke, The One and the Many (Notre Dame, Indiana: University of Notre Dame Press, 2001), 160. Kindle.
 Francis Selman, Aquinas 101 (Notre Dame, Indiana: Ave Maria, 2007), 208.
 Clarke, The One, 41.
 Thomas Aquinas, “Of God and His Creatures,” LXXV ad 2, in Saint Thomas Aquinas Collection (Aeterna Press, 2016), 6600. Kindle.
 Thomas Aquinas, Summa theologiae, I, q. 75, a. 2 ad 1, at New Advent, www.newadvent.org.
 ST I, q. 76, a. 1.
 ST I-II, q. 3, a. 5.
 ST I, q. 91, a. 3.
 ST I, q. 79, a. 4.
 Clarke, The One, 336.
 Thomas Aquinas, “The Soul,” a. 14, in Saint Thomas Aquinas Collection (Aeterna Press, 2016), 8321. Kindle.
 ST I, q. 75, a. 2.
 ST I, q. 75, a. 3 ad 2.
 ST I, q. 85, a. 2.
 Edward Feser, Aquinas (London: Oneworld, 2015), 145.
 Feser, Aquinas, 153.
 Feser, Aquinas, 145.
 ST I, q. 85, a. 2; cf. ST I, q. 75, a. 2 ad 3.
 Feser, Aquinas, 156.
 ST I, q. 75, a. 6.
 Selman, Aquinas 101, 96.
 ST I, q. 75, a. 4 ad 2.
 Feser, Aquinas, 161.
 Selman, Aquinas 101, 94.
 ST I, q. 1, a. 1.
 Catechism of the Catholic Church, 2nd ed. (Washington, DC: United States Catholic Conference, 2020), 998. Kindle.
 Selman, Aquinas 101, 181.
 Clarke, The One, 132.
 W. Norris Clarke, Person and Being (Milwaukee, Wisconsin: Marquette University, 2006), 35.
 Selman, Aquinas 101, 97.