To be more precise, what is a human soul? Or to be even more precise, what is a human being? For that is really the key question; and I sometimes think that the biggest obstacle to understanding what the soul is is the word “soul.” People too readily read into it various erroneous notions (erroneous from an Aristotelian-Thomistic point of view, anyway) -- ghosts, ectoplasm, or Cartesian immaterial substances. Even the Aristotelian characterization of the soul as the form of the living body can too easily mislead. When those unfamiliar with Aristotelian metaphysics hear “form,” they are probably tempted to think in terms of shape or a configuration of parts, which is totally wrong. Or perhaps they think of it in Platonic terms, as an abstract universal that the individual human being participates in -- also totally wrong. Or they suspect that since it is the form of the living body it cannot coherently be said to subsist apart from that body -- totally wrong again. So let us, for the moment, put out of our minds all of these ideas and start instead with the question I raised above. What is a human being?
To ask what a human being is is to ask what the nature of a human being is. What makes human beings the kinds of things they are? What makes them distinctive? What sets them apart from other kinds of thing? To answer this it is useful to consider those kinds of thing which, on the Aristotelian-Thomistic view, come just below and just above human beings in the hierarchy of reality: non-human animals, and angels.
An animal is something which by its nature not only exercises vegetative powers like taking in nutrients, growing, and reproducing, but is also capable of sensation and imagination, of appetite, and of locomotion or the ability to move itself in response to the promptings of appetite and in pursuit of what it senses or imagines. Particular kinds of animals will, given their natures, exhibit this repertoire in their own distinctive ways. For instance, land animals will exercise their locomotive powers by walking, hopping, or slithering, fish by swimming, and (most) birds by flying; and each will do so by means of its own distinctive organs -- legs, fins, wings, and so forth.
Now of course, not every single individual animal will perfectly exercise the capacities that are natural to it, or even actually possess the organs that are its natural means of exercising them. A dog might injure or lose a leg, or even fail to develop legs in the first place because of some prenatal defect. But it is still of the nature of such a dog to have legs, and to walk and run with them. In the extreme case, we can even imagine a dog which (as a result of an accident, say) has lost not only its legs, but its sense organs and higher brain functions, and is kept alive through intravenous feeding -- reduced, in effect, to a portion of its vegetative functions. All the same, the nature of such a dog, no less than that of a healthy dog, is to have sense organs, legs, and all the rest. That the dog has been prevented from realizing that nature doesn’t change the nature itself; and should the dog be somehow restored to health and functionality, it is precisely those doglike attributes that it had lost that would be restored to it, rather than some other attributes.
Consider now an angel, which stands on the other side of the metaphysical divide marked by human beings. An angel is, by nature, a creature of pure intellect, which entails -- given that, as Aristotelian-Thomistic philosophers argue, intellect is necessarily immaterial -- that an angel is essentially immaterial. (The wings, white robes, and long blonde hair are symbolic -- suitable for children’s prayer books but not for metaphysics!) Being immaterial, angels cannot be damaged or physically malformed the way an animal can. (Of course, angels can be morally defective -- there are fallen angels, after all -- but that is a failure of will, which is an immaterial power that follows upon intellect.) Indeed, being immaterial, they have no tendency toward corruption at all. They are of their nature immortal.
And now we come to human beings. A human being is by nature a rational animal. That is to say, a human being is something which by its nature exercises both the animal powers of nutrition, growth, reproduction, sensation, appetite, and locomotion, and the intellectual and volitional powers possessed by angels. Hence it exercises powers of both a material and an immaterial sort. For that reason it is to a large extent capable of damage and malformation, as an animal is; but not completely so. In particular, a human being can be damaged to such an extent that it completely loses the organs of its animal and vegetative powers, and thus cannot exercise them at all -- to such an extent that only its intellectual and volitional powers remain. But those intellectual and volitional aspects of human nature, precisely because they are immaterial and thus do not depend on any corruptible material organ, cannot themselves perish, any more than they can in the case of an angel -- though they would be impaired given that the human intellect’s normal source of data is the sense organs, which are material, and given that its activity is normally carried out in conjunction with imagination, which is also material.
Now what we’d have in the case of a dog which had lost its legs, its sense organs, and its higher brain functions is the stub of a dog, the bare minimum consistent with the dog’s surviving at all. The nature of such a poor creature would not have changed, but it would have been reduced to realizing only the smallest fragment of what would naturally flow from that nature. You might almost say that it had been reduced to little more than the nature itself, with almost nothing in the way of a manifestation of that nature. And a human being damaged to such an extent that it could exercise none of its animal capacities and retained only its intellectual and volitional faculties in an impaired state would, you might say, be a stub of a human being, the bare minimum consistent with a human being’s surviving at all -- a human being reduced to little more than its nature, with almost nothing in the way of a manifestation of that nature. The key difference would be that whereas the severely damaged dog of our example could also go on utterly to perish, this stub of a human being could not. It is immortal, though the full human being is not, which is why resurrection is necessary. (To be sure, God could annihilate this “stub,” just as He could annihilate anything; but as with an angel, nothing in the natural order could destroy it, because, being immaterial, it would have no inherent tendency toward corruption.)
Now such a stub of a human being is what a soul is, or a disembodied soul anyway. This is why Aristotelian-Thomistic philosophers often call a disembodied soul an “incomplete substance” -- not because they are trying incoherently to fudge the difference between a Cartesian res cogitans and the idea of the soul as a kind of form, but because a disembodied soul relative to a living human being is like a legless, senseless, brain-damaged dog relative to a healthy dog. The severely damaged dog is in an obvious and natural sense an incomplete substance, and the disembodied soul is an incomplete substance in just that sense -- it is an incomplete, damaged human being.
This is also a way to understand the sense in which the soul is the substantial form -- that is to say, the nature -- of a human being. A nature or substantial form is not a Platonic abstraction. It exists in a concrete individual thing, as its principle of operation and the source of its properties. It is there as long as, and only as long as, the individual thing itself is there. But when the operations and properties in question are prevented from being manifested, what we are left with in effect is the principle or source without that which flows from it. Thus to reduce a human being to the bare minimum consistent with its being there at all is to reduce it as far as possible to its nature or substantial form -- that is, to its soul alone.
Some might insist that if the intellectual and volitional powers of a human being persist in even an impaired form after the animal powers have been destroyed, this must be because the former inhere in a substance distinct from that in which the latter inhere, as Descartes held. But this is like saying that since the stub of a dog would continue to exist in the absence of its legs, eyes, ears, etc., it follows that the stub in question (an eyeless, earless, brain-damaged torso) and the legs, eyes, ears, etc. are all distinct substances. And they are not; rather, they are all aspects of one substance -- the dog itself -- and can be made sense of only by reference to that one substance. Similarly, that the impaired intellectual-cum-volitional stub of a human being would continue to exist in the absence of its animal powers does not entail that the stub in question and the animal powers must be grounded in distinct substances. They are not; rather, they too are aspects of the one substance -- the human being himself -- and can be made sense of only by reference to that one substance.
I noted in a recent post that those beholden to scientism tend to reify abstractions -- to abstract the mathematical structure of a concrete physical system and treat it as if it were the entirety of the system, or to abstract the neurobiological processes underlying human action and treat them as if they were the whole source of human action. I also noted that while those prone to scientism are notorious for this, Cartesians are guilty of reifying abstractions too. Specifically, they abstract from the one substance that is a human being its intellectual aspect and its animal aspect and make of them two substances -- putting asunder, as it were, what God and nature had joined together. And when they finally recombine them, what they are left with is nothing human at all, but a bizarre shotgun marriage of angel and animal, or ghost and machine. But sometimes a man is just a man.