The contributions of St. Thomas Aquinas to the development of Catholic theology cannot be overestimated. He was a genius, and although not the first, he was, arguably, the greatest from the point of a view of sheer mental power. This much is generally acknowledged, but we would like to provide a few preliminary remarks that should be kept in mind when we look at his doctrine of the soul.
First, when a terminology gains currency, and if it is proven both effective and powerful, it can become the de facto norm, even if it is not, strictly speaking, the only way of enunciating the truth. In other words, even if a certain formulation (“the soul is the form of the body”) is not in and of itself definitive, it can become effectively irreplaceable once enmeshed in the official theological vocabulary.
Pope Pius X, in the 1914 encyclical Doctoris Angelici, says that this is precisely the case with St. Thomas:
The capital theses in the philosophy of St. Thomas are not to be placed in the category of opinions capable of being debated one way or another, but are to be considered as the foundations upon which the whole science of natural and divine things is based; if such principles are once removed or in any way impaired, it must necessarily follow that students of the sacred sciences will ultimately fail to perceive so much as the meaning of the words in which the dogmas of divine revelation are proposed by the magistracy of the Church.
In other words, Thomism is not necessarily identical with Catholic theology, but is now practically identical with it. This for the most part reasonable and well-deserved, since no other theologian has provided a framework so reliable and unassailable when it comes to a dialectical exposition of the faith.
We repeat: this is well-deserved, and so nothing of what we say here is intended to disparage St. Thomas or minimize his importance. We only wish to make clear certain dangers that come with this situation, the foremost of which is that it seemingly absolutizes a certain point of view that is not absolute. Once absolutized, it excludes all other possible enunciations, not only some of those offered in the thousand years prior to Thomism, but (and this is far more alarming) any that may possibly be offered in the indeterminate future.
To refer to another example of the same phenomenon, where a certain approach becomes so popular as to become artificially normative, we have elsewhere discussed how the mysticism of Teresa of Avila and John of the Cross, profound and wonderful as it is, came to be seen as the only style of spiritual realization compatible with Catholicism. Anyone whose spiritual temperament does not coincide with the style of the two great mystics of 16th century Spain are given the impression that either they are somehow in error, or else that Catholicism has nothing to offer them in the form of spiritual method.
These same drawbacks, mutatis mutandis, occurred when Thomism became the normative theological vocabulary in the Catholic world.
With that said, we will briefly summarize the way in which St. Thomas envisages the soul.
There are, it is said, three souls (or ‘parts’ of the soul): the vegetative, the animal, and the intellectual.
As for the first two ‘souls’, they are a result of the generative process itself, which is to say they are not created directly by God but are produced from preexisting psychic or subtle substance in a similar way to how the body itself is not created directly but is formed from the existing material environment.
So far, this gives a certain unity to the human person and fits into the standard body-soul dualism. But what about the intellectual soul? For Aquinas, as for us, it is clearly something that is, by its very nature, cannot be situated in the psychic order, nor can it be a result of the generative act. Did not Aristotle say that it was something that comes from without, “through the door” of the heart? Aquinas agrees, and must allow for its presence within a fundamentally Aristotlean terminology but without compromising the unity of the soul.
His solution is that at its earliest stage, for example as an embryo, the human being is given form by the various souls in stages and, additionally, as the being moves from one stage to the next (for example, from vegetative to animal) the soul does not develop but is instead “replaced” by the higher type of soul, which carries on the functions of the lower type even while replacing it, since the higher can accomplish what the lower can accomplish.
This allows Thomism to account for the third soul, which is possessed of something categorically different than the two lower souls: mentality.
The explanation runs as follows:
“Thus the vegetative soul, which is present first (when the embryo lives the life of a plant), perishes and is succeeded by a more perfect soul, both nutritive and sensitive in character, and then the embryo lives an animal life; and when that passes away it is succeeded by the rational soul introduced from without, while the preceding souls existing in virtue of semen.”
We mentioned already that the Thomistic solution potentially raises more difficulties than it resolves. For example, if the rational soul performs a function categorically different from the two previous souls, which it is said to replace, its unity seems contrived from the start. It retains the ability to perform the functions of the lower souls, which were related to the organization of the body, but the power of the intellect is not tied to the body and is in fact separable from it. According to Aquinas himself:
“A body is not necessary to the intellectual soul by reason of its intellectual operation considered as such; but on account of the sensitive power.”
The soul is called “the form of the body” in terms of its vegetative and animal functions, which is clear enough with regard to the lower two souls; but the third and most perfect soul, created by God and not generated through nature, performs a function that is radically different from what the soul had, until that moment, performed, and brings with it a new power, the intellect, which is separable from the body, even though in its vegetative and sensitive functions, the soul is inseparable from it.
It is hard not to suspect that we are dealing with the same old spirit-soul-body anthropology, except that, due to the historical challenges faced by Aquinas, as well as his thoroughly Aristotelian vocabulary, it is veiled behind a powerfully rationalized dualism. We do not, for all that, contest its value or orthodoxy; what we do contest is the wisdom of insisting on it as the end-all-be-all of anthropological formulations.
In sum, we would say that whatever the merits of Thomism, they do not disqualify the merits of Irenaeus, Clement, Augustine, Origen, Jerome, and others; and it would be more beneficial if Catholic scholarship would, perhaps without even abandoning Thomism as a preferred standard, discontinue its unofficial monopoly on doctrinal expression.
 Summa Contra Gentile, Bk. II, ch. 89.
 Summa Theologiae I, q. 76, a. 5, ad. 2.