Dr. Michael Augros: “Aquinas’s Proof for the Human Soul’s Subsistence”

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by Dr. Michael Augros
Tutor, Thomas Aquinas College, New England
Keynote Address
2023 Thomistic Summer Conference
June 17, 2023

 

So vast is the soul, said Heraclitus, that one could explore it in all directions and never come to the end of it.[1] That is true not only of soul in general, but also of the human soul in particular, and even of the still-more-particular investigation into its immortality—one can spend a lifetime learning about it. St. Thomas infers its immortality, however, from its subsistence,[2] since that is prior[3] to the human soul’s natural incapacity to cease existing. The question whether the human soul subsists, therefore, is of profound philosophical importance.

Understanding its subsistence is necessary also for understanding what the human soul is. If we do not understand a thing’s natural mode of existence, then we do not understand its nature, since its existence is the actuality of its essence. If we do not know whether heat or light exists in the mode of a substance or an accident, for example, we do not yet know what heat or light is. Even after learning that souls in general are the substantial forms of corporeal living things, we do not yet fully understand what the human soul is. By that general definition of soul we know that the human soul doesn’t exist in the mode of an accident, but we do not yet see that it also doesn’t exist in the same mode as any other soul or substantial form.

Other souls and substantial forms St. Thomas calls “material forms,”[4] forms of matter that do not have their own existence, but are only that by which some material being has existence.[5] Not having any existence of their own, such souls cannot exist apart from the material beings they constitute, and so St. Thomas says they are “immersed in matter”[6] and that they do not subsist through themselves. By contrast, the human soul does have its own existence, and is not merely that by which something else (a complete human being) exists. To express this, St. Thomas says that the human soul is subsistent, and that it is a hoc aliquid.[7] It has an existence that it can share with matter[8] so as to constitute a complete human being, but which it need not share with matter. One could say this means that the human soul alone, of all souls, is not just a soul but is also a spirit—just as it alone, of all the spirits, is also a soul.

Answering the question whether the soul is subsistent is thus necessary in order to understand what it is. For that reason St. Thomas in his discussion of the soul in the Prima pars shows first that it is the actuality of the body and then that it is subsistent,[9] and in his Disputed Question on the Soul asks first whether the soul is a hoc aliquid, which he answers by showing it is subsistent.[10]

Aristotle, too, says that we must come to understand what the soul is through its accidents (that is, through its powers, acts, and undergoings), and the foremost question about these is whether any of them belong to the soul alone or do they all belong also to the body.[11] He ends up affirming that the soul’s act and power of understanding is the soul’s alone, so that the soul has an existence of its own as well. He says, in other words, that the human soul is subsistent, and he arrives at that conclusion by an argument that St. Thomas adopts after him.

The thesis of this essay is that St. Thomas’s proof of the subsistence of the human soul is a demonstration. To make that as clear as possible in a brief space, I will first present his argument, then offer a sample of its power to withstand objections, and finally describe the argument’s demonstrative nature.

 

St. Thomas’s Argument

First, here is the argument itself, as he formulates it in his Prima pars:

It is necessary to say that that which is the principle of intellective operation, which we call the soul of man, is an incorporeal and subsisting principle. For it is clear that through intellect man is able to know the natures of all bodies. Now it is necessary for what is able to know certain things to have none of them in its nature, because whatever would be in it naturally would prevent knowledge of the others; we see, for example, that a sick person’s tongue, saturated with a choleric and bitter humor, cannot perceive something sweet, since instead all things seem bitter to it. So, if the intellective principle had within itself the nature of a body, it would not be able to know all bodies. For every body has a determinate nature. Therefore, it is impossible for the intellective principle to be a body. And similarly it is impossible for it to understand through a bodily organ, because the determinate nature of that bodily organ, too, would prevent the knowledge of other bodies, just as a liquid poured into a glass vase would appear to be of the same determinate color not only as one existing in the pupil, but also as one that is in the glass. Therefore, the intellective principle (which is called the mind or the intellect) has an operation by itself which it does not share with the body. Now nothing is able to operate by itself unless it subsists by itself (since operating belongs only to something actually existing; hence something operates in the mode in which it exists). For this reason, we do not say that heat, but a hot thing, heats. It remains, then, that the human soul (which is called an intellect or a mind) is something incorporeal and subsisting.[12]

The main argument, here, is simply this first figure syllogism:

(Major)  What has an operation of its own has an existence of its own (that is, it subsists).

(Minor)  The human soul has an operation of its own (namely, understanding).

(Conc.)  Therefore, the human soul has an existence of its own (that is, it subsists).

Both the major and the minor are supported. The major is just a particular case of a universal principle, namely that only what has actual existence can act or operate. Because that is so, only what has its own actual existence can have its own operation. The alternative would be for something to have its own operation yet not have its own existence—for sharpness, say, to cut things by itself, even though it is the sharpness of a knife. Of course that is impossible. If sharpness does not exist by itself, but is something of a blade or knife, then neither can it cut by itself; for it to cut an apple is really for the knife to cut the apple through its sharpness. So, only what has its own existence, or what subsists, can have its own operation. So much for the major.

Much more difficult to grasp, and far more controversial, is the minor premise: the human soul has an operation all its own, not shared with the body, namely understanding. St. Thomas proves this by an argument due originally to Aristotle.[13] This crucial supporting argument can be presented as a single syllogism reasoning modus tollens, as follows:

If the human intellect were either a part of the body or a power existing in and operating by means of one (say, a portion of the brain), then it would naturally possess its own corporeal nature, would consequently always be knowing that one determinate nature, and would thus be unable to know all other corporeal natures.

But the human intellect is not always thinking about some one determinate corporeal nature, and is able to know all corporeal natures.

Therefore, the human intellect is not a part of the body, nor does it reside in and operate by means of a part of the body.

 

An Objection to St. Thomas’s Argument for the Minor

Though this argument is not the whole of St. Thomas’s proof for the human soul’s subsistence, but only the supporting argument for that proof’s minor premise, it is the true heart of his proof, and so I will focus on it. Most of those who are familiar with Aristotle or St. Thomas are familiar with this argument. But familiarity and complete understanding are not the same; in my own philosophical development I am still somewhere between mere familiarity with this argument and perfect understanding of it. Perfect understanding of this sort of argument requires the ability to formulate and resolve all the powerful counterarguments that can be brought against its premises, or against its way of reasoning, or against its conclusion. Many counterarguments can be made against any of St. Thomas’s demonstrations that the human intellect is incorporeal, but that is especially true of this one from Aristotle. By my count, the would-be refutations of it number over a dozen. In the interests of brevity, I will discuss just one. In the interests of making a small contribution to the understanding of the argument, the one would-be refutation of it that I will discuss here is one that is not easily answered, one that is nowhere considered by St. Thomas himself, one that is often made by his critics today and yet is seldom answered by his living disciples.

The counterargument I have in mind says that St. Thomas’s if-then premise is false. The premise, recall, states that if the human intellect were corporeal, then it would be stuck knowing that one corporeal nature all the time, which would interfere with its knowing other corporeal natures. However (says this would-be refuter), it does not follow from the supposition that the mind physically possesses a corporeal nature that it must know that nature. It is not a general rule that a cognitive organ knows its own physical properties, not even when they are its objects.

The visual cortex within the brain, for example, physically possesses certain colors which are its objects, but it does not see these colors that physically belong to it. Nor does the part of the brain responsible for touch feel its own pressures and temperatures, though these are physically present within it. It is not physical possession of an object that causes a cognitive power to know it, then, but instead another sort of possession of the object causes knowledge of it, namely possession of the object by way of a cognitive representation. We know an object not by physical possession of it, but by having a sensible or intelligible species of it.

It is strange that St. Thomas has forgotten this (continues the objector), since he himself often insists that all knowledge is through a likeness, or through an assimilation,[14] that is, through the likeness or species of the object existing in the knower, not through the object itself in its own natural existence. Hence he says, in agreement with Aristotle, that “stone does not exist in the soul” that knows what stone is, “but the species of stone”[15] exists there. In other words, not an actual, physical stone itself, but some likeness or representation of stone, exists in the mind that understands it.

We can now appreciate the force of the objection. St. Thomas says that if the human intellect had a corporeal nature, then just by that fact it would know that nature. But in reality knowledge of its own corporeal nature would follow only if that nature is assumed to exist in the intellect with a representational and intelligible existence, not if it is assumed to exist there only with a natural, physical one. If we assume the intellect is a corporeal power, but not that it is always in possession of an idea of itself, then it does not follow that it always knows itself, or that consequently it cannot know other things. St. Thomas has shown only that the human intellect must not natively possess any intelligible representation of itself, but nothing in his argument prevents the intellect from having its own corporeal nature in a physical way, just like any other cognitive power we possess. In this way, the argument seems fatally flawed.[16]

 

How St. Thomas Would Resolve the Objection

To my knowledge, St. Thomas nowhere raises this objection to his argument. He nonetheless supplies us with all we need to answer it, as I hope now to show.

The objector relies on the idea that all knowledge proceeds from a representation that is really distinct from the form of the object in its own natural existence, and even claims that St. Thomas agrees with this. But he does not agree. He does sometimes say that all knowledge is through the likeness of the known in the knower, or through assimilation, but when he does so he means to include cases in which the form through which the knower knows is just the form of the object in its own natural existence—as happens in divine and angelic self-knowledge, and in any creature’s vision of the divine essence.[17] In such cases the form within the knower is “like” the form of the known in the understated sense in which a thing is “like itself.” Other times, however, St. Thomas uses distinct expressions for talking about these two cases, saying that some knowledge is per similitudinem (that is, through a likeness really distinct from the form of the object in its own natural existence), whereas other knowledge is per essentiam (that is, through the form of the object in its own existence).[18]

St. Thomas gives two signs or indications that knowledge must sometimes be per similitudinem, and also gives the reason why it is necessary when it is necessary. The first sign that knowledge must at least sometimes be per similitudinem is that otherwise natural similarity would always suffice for knowledge, and then there would be no reason why one stone would not know another, or why fire would not know fire.[19] Another sign is that if natural similarity were the sufficient and universal cause of knowledge, then the sense of sight would know hearing instead of color, since its nature is more like hearing than like a color,[20] and similarly the senses would know bodies better than the intellect does,[21] whereas these things are false. Hence knowledge must often take place through a certain likeness of the object’s form rather than through such a form in its own natural existence.

St. Thomas also explains why this so. The reason is that knowers are meant to know things other than themselves, since it is the proper perfection of knowers to be capable of having the forms of other things.[22] Obviously, the forms of determinate things other than the knower cannot exist in the knower with their own natural existence;[23] sticks and stones and cats and dogs cannot exist in the human soul with their own natural existence, for example. Hence knowledge through a likeness, rather than through the form of the object in its natural existence, is required in such cases.

But this is not a reason why all knowledge whatsoever must be per similitudinem. Rather, this reason applies only to cases in which a knower knows primary and immediate[24] objects that are other than the knower himself. Hence it does not show that a hypothetically corporeal human intellect must know itself per similitudinem, since that is a case of self-knowledge.

In sum, the would-be refutation falsely assumes, without supplying a reason, that knowledge in all cases follows upon possession of the object’s form not in its natural existence. And the reason St. Thomas himself supplies for knowledge needing to be per similitudinem applies only to knowers knowing things other than themselves, and so it cannot apply to a hypothetically corporeal intellect knowing itself. Thus does the refutation fail to overturn St. Thomas’s if-then premise.

On the other hand, the failure of an attempt to show St. Thomas’s if-then premise is false does not imply that it is true. And even if the would-be refutation has failed to prove it false, it has drawn attention to the fact that it is a strange if-then statement, one whose truth is not immediately clear.

Why, then, must we admit that on the hypothesis that the human intellect is the brain (or some portion thereof) it follows that it knows its own nature all the time? The answer comes from St. Thomas’s very argument, which posits that the primary object of the human intellect is “the natures of all bodies.” Given that the human intellect is itself a body or a corporeal nature in a part of the body, such as the brain, it follows that it is one of its own primary objects. Consequently, it will know itself, since all that is required for knowledge is for a cognitive power to fully possess the form of one of its primary and immediate objects.[25]

Two questions arise, here. First, how can we be sure that knowledge requires nothing more than the existence of the object in the cognitive power? Might not some special mode of existence also be required? Second, if there is nothing more required, then why doesn’t the part of the brain responsible for the sense of touch always feel its own tangible qualities, and why doesn’t the visual cortex always see its own visible qualities?

Consider the first question first: why is it enough for the object to exist somehow in the corresponding cognitive power? Why don’t we have to add “with the appropriate mode of existence capable of causing knowledge”?

The answer is that the entire reason why a special mode of existence of the object’s form, other than its own natural existence, is sometimes required for knowledge, is that the object is other than the cognitive power, and so it cannot exist in that power with its own natural existence. It must exist there, instead, through a representation whose natural existence complies with the exigencies of such a cognitive power. For example, if the cognitive power is sight, which must exist in eyes and brain and not in stone, then there is no way to get the form of stone into sight with its own natural existence; instead, it must exist there as a visual representation which can have its natural existence in eyes and brain.

The primary and universal requirement for knowledge is for the object[26] to exist in the cognitive power. But whatever exists in something must exist in it through the mode or capacity of the thing it is in.[27] Therefore, this secondary and particular requirement for knowledge must follow: whenever the object cannot exist in the cognitive power with its own natural existence, knowledge will require it to exist there with some other sort of existence compatible with being in that power.

But not otherwise. If a primary and immediate object exists in a cognitive power with its own natural existence, that short-circuits any need for it to exist there in some other mode in order for knowledge to follow. Hence St. Thomas holds that knowledge does in some cases follow on the natural existence of the thing known within the knower. When asking whether an angel sees the divine essence through a created likeness of that essence existing in the angelic mind, St. Thomas entertains the objection that all knowledge is by assimilation, hence by means of some likeness of the known existing in the knower. He answers it in this way:

Knowledge does not require assimilation except in order for the knower to be in some way united with the thing known. And a union by which the thing itself, through its very essence, is united to an intellect is greater than if it were united by its likeness. And so, since the divine essence is united to the intellect of an angel as its form, it is not required, in order for [the angelic intellect] to know that [essence], for it to be informed by a likeness of it, so that, by means of this [likeness] coming in between, it would know it.[28]

Hence a hypothetically corporeal human intellect would not need to be informed with a representation of itself, so that, by means of this intermediate likeness, it could know itself. Instead, its form and nature in its own natural existence would already exist in it, and that would cause it to know itself.

So much for the first question. Consider now the second. Does St. Thomas’s if-then premise oblige us to say that the sense of sight or imagination must always see certain colors, since the part of the brain housing such powers naturally possesses certain colors?

Not at all. The natural existence of colors (or temperatures, and so on) in the brain does not unite them with the cognitive powers in the brain. Though their organs have natural colors, neither sight nor imagination itself consists in being a certain color; by contrast, intellect itself would consist in being a certain corporeal nature if it were a power of the brain. If, analogously, the power of sight itself were the color pink, for instance, then indeed the sense of sight would always see pink. So too, if the intellect were a certain corporeal nature, then it would always understand itself.

Moreover, it is not the entire organ of an external sense that is receptive to its object in its natural existence, or to its medium. Instead, one part of the organ is dedicated to being thus receptive, while another part is more active (though St. Thomas was not aware of this),[29] working up the received information into a complete sensory representation of the object. Tactile sensation of what the hand is touching, for instance, is completed in the brain, not in the hand, but what is thus felt is in the hand, not in the brain. The touch centers within the brain have no way of receiving tactile information immediately from the tangible qualities materially existing in the brain itself. Likewise, the seeing of an object whose form enters through the eyes via light takes place in the brain, not in the eyes or even in the optic nerves,[30] but what is thus seen is the external object, not something in the brain. And it is only the receptive part of the organ of external sense that would affect sensation by physically possessing its own object.

For example, it is not all parts of the complete organ of sight, but only certain parts of the eye, that are receptive to light and color in its natural being. And so only in certain parts must it lack color—in the cornea, aqueous humor, lens, and vitreous humor.[31] If the eye physically possessed a lit color in one of these places, it would always be seeing that color. As things are, it is possible for a body to lack all colors, to be perfectly or nearly transparent, and so the organ of sight takes advantage of that, putting only transparent materials between the retina and the incoming light from the object, so that we do not find ourselves always seeing one set of colors naturally existing in our own eyes.

Touch is a different case, since it is impossible for a body to lack all tangible qualities, such as temperatures and pressures. Consequently, touch has no choice but to allow the temperature of its own medium—human tissue—to become a component in all its temperature sensations of outside objects. That of course fits with experience; which of two bowls of water feels warmer to our two hands, for example, is determined in part by the temperatures of the water in the bowls, but also by the temperatures of our hands. This is one way, then, in which touch respects St. Thomas’s rule that a cognitive power receptive of certain determinate objects must lack them in its own nature: touch in some way does always know its own temperature, and this does interfere with, or restrict, its knowing the temperatures of other objects.[32]

Like tangible qualities, corporeal natures cannot be entirely absent from a corporeal organ. Hence if the intellect were some part of the brain, every part of it would have a determinate bodily nature. Then the intellect itself, already being one of its proper and primary objects, would bypass the need for any intelligible representation through which to make its own corporeal nature present to itself. Therefore, the intellect would always be in full possession of one of its own primary and proper objects, and consequently would always know itself, just as St. Thomas says. Its own specific nature would enter into every one of its thoughts. But that is contrary to experience. We can think of corporeal natures entirely other than that of the human brain, apart from any relation they have to it.[33]

 

Logical Classification of St. Thomas’s Arguments

As promised, that counter-argument is the only one taken up here. Let us now move on to a consideration of the logical nature of St. Thomas’s argument for the subsistence of the human soul.

It is a demonstration, since one premise (viz., “what has its own operation has its own existence”) follows immediately from things that are self-evident, and the other (viz., “the soul has its own operation”) is itself demonstrated. More specifically, the argument is a demonstration quia, a demonstration that its conclusion is true. It is not telling us why the human soul subsists. That it has its own operation does not cause it to subsist. Rather, that it subsists is why it has its own operation.[34]

It is in this way similar to the argument for God’s existence from motion. The whole argument is a demonstration, but quia; it is not showing us why God exists, but that he exists, reasoning from motion, whose existence is evident to us, to the ultimate reason why it exists. Still, that argument is based on the crucial premise that motion needs a mover, which can be demonstrated propter quid. What about St. Thomas’s supporting argument for his crucial premise that the human intellect is not a part of the body? Is that a demonstration? And is it demonstration quia, or propter quid?

We will be able to tell more easily whether this argument is a demonstration if we reformulate it as a categorical syllogism, as follows:

1. A power of receiving and knowing all corporeal natures must lack corporeal nature;
2. The human intellect is a power of receiving and knowing all corporeal natures;
3. Therefore, the human intellect must lack corporeal nature.

The major premise is self-evident, or borders on something self-evident: what is to receive and thereby know all things of certain determinate natures must lack them all, since a thing cannot already have what it is to receive.[35] Nor can it naturally possess one of the things in a whole genus of things it is to receive, and lack only the others, if the one thing would prevent it from receiving all the others. This is the case with corporeal natures.[36] Having one corporeal nature does not permit receiving others (e.g., being a cat prevents something from receiving the nature of a dog), and knowing one corporeal nature through an intelligible species dedicated to it[37] does not permit knowing other natures not represented through that species (e.g., simple grasping of what a cat is, through a species of cat, does not enable, but prevents, simple grasping of what a dog is).

The minor premise, too, is self-evident, and is made known through experience. Hence St. Thomas says it is “clear that man through intellect can know the natures of all bodies.”[38] By definition the human intellect is a power of receiving and knowing all corporeal natures. And it is a matter of experience that we are in possession of such a power. It is a matter of experience that the human mind can grasp all bodies in general, say, by forming a definition of body that applies to all of them, and also by making true statements about them all, as we see it does both in the case of all natural bodies (as in physics) and also in the case of all mathematical bodies (as in geometry). It is also a matter of experience that the human mind can grasp all bodies in their common principles, since it can understand the chemical elements or ultimate particles that compose them all. It is also a matter of experience that the human mind can grasp all bodies in their specific natures at least to some extent, since these become knowable to it through experience of individuals, from which it learns both their common genus and the differences that divide it, and also their common materials and the special compositions that distinguish them.

St. Thomas’s argument for the premise that the human soul has its own operation, then, is indeed a demonstration of some sort. But is it demonstration quia, or propter quid? Comparison with a similar argument about an easier matter may help to decide. Consider this argument about the aqueous humor of the human eye:

1. A medium receptive to all colors must lack color;
2. The aqueous humor is a medium receptive to all colors;
C. Therefore, the aqueous humor must lack color.

Here the middle term is “receptive to all colors.” Is that an effect, or a cause, of the fact that the aqueous humor lacks color?

If we refer to material causation, it’s an effect. The humor is receptive of all colors because it lacks them all. On this understanding, “because” means “due to the following disposition in the humor, rendering it apt to receive.” In a similar sense, a movie screen is receptive of projected images “because” it has no image on itself that would interfere with those images. Similarly, prime matter is receptive of all forms “because” it lacks them all in its own nature. Of course, prime matter receives materially, whereas the aqueous humor does not.[39] Still, the dispositions required for its special kind of receptivity are more like material causes of its receptivity than they are like any of the other three genera of causes. Understood analogously, the argument for the intellect’s incorporeal nature is effect-to-cause reasoning, hence demonstration quia: because the intellect lacks all corporeal natures, it is tied to none, and is capable of understanding them all. The conclusion gives the quasi-material[40] cause of the truth of the minor premise that the intellect can understand all things.

But what if we refer instead to final causation, and ask again whether being receptive to all colors is an effect, or a cause, of the fact that the aqueous humor lacks color? In this way, it is a cause. The aqueous humor lacks all colors because it receives them all. Here, “because” means “due to its being for the sake of the following end.” In a similar sense, a movie screen has no image on itself “because” it is meant to receive projected images. And prime matter lacks all forms “because” it is to receive them all. Understood analogously, the argument for the intellect’s incorporeal nature is cause-to-effect reasoning, hence demonstration propter quid.

Which way of taking the argument is correct? They are both correct. It would be a mistake only to think that the mind’s receptivity to all corporeal natures is both a cause and an effect of its incorporeality in the same genus of causation. Its universal receptivity is an effect of its incorporeality in the line of quasi-material causation, but a cause of it in the line of final causation.

In which way did St. Thomas intend his argument to be taken? The language of the argument itself[41] seems indeterminate, open to either way of being understood. But St. Thomas quite clearly holds that the acts and objects to which powers are ordered are their final causes. Here is what he says:

… Powers [of the soul] are differentiated by [their] acts and objects. Now some say that this should not be understood in the sense that the diversity of acts and objects is the cause of the diversity of powers, but only in the sense that it is a sign of it. Others say that the diversity of objects is the cause of the diversity of powers in the case of passive powers, but not in the case of active ones. However, if one considers the matter carefully, one finds that in the case of both kinds of powers their acts and their objects are not only signs of their diversity, but are in some way the causes of it. For everything whose existence is only on account of some end has a mode determined for it from the end to which it is ordered. A saw, for example, both in its material and in its form, is of such a sort as to be suitable for its end, which is to cut. And every power of the soul, whether active or passive, is ordered to its act as to an end, as is plain from Metaphysics 9. Hence each and every power has a determined mode and species as a consequence of which it can be suitable for such and such an act.[42]

Accordingly, as the eye’s transparency is for the sake of receiving and seeing all colors, so is the intellect’s incorporeality for the sake of receiving and understanding all corporeal natures. The intellect must not be composed of corporeal principles, or combined with any of them, in order that it may know them all.

The argument reasons necessarily, and does so from necessary truths that contain the proper cause of the truth of the conclusion. It is, then, a philosophical demonstration in the fullest sense.

 

Conclusion

And yet it also seems to be somehow first for us. It is prior to other demonstrations for the incorporeality of the human intellect. One sign of this is that Aristotle gives no other demonstration of the human intellect’s incorporeal nature, and none existed, or not fully fledged, prior to him.

Another sign of its primary character is that St. Thomas prefers it when addressing “beginners” in sacred theology. In his Summa contra Gentiles, which is not for beginners, he offers many other arguments for the incorporeality and immateriality of intellect.[43] In the Summa theologiae, he offers only one argument for the incorporeality of intellect on the way to proving the human soul’s subsistence, namely the one from Aristotle.

Probably the reason it is somehow first for us is that it argues from the things that the soul somehow has in common with the senses and even with matter, namely receptivity to corporeal things. Though the intellect does not “receive form” in the same way, or even in the same sense, as matter does, or as the senses do, it does receive form, and in that way it is like sense.[44] The argument begins with what the intellect has most in common with the senses, and in that way begins with what is most known to us about the intellect. Like the senses, and most especially like sight,[45] the intellect must lack its primary objects in its own constitution. Unlike sight, intellect has all corporeal natures as its primary objects. From these facts it follows that unlike the senses, the human intellect has no corporeal organ. It is most natural for reason to proceed from the sensible to the intelligible—and here we have a particularly striking example of this, since Aristotle’s argument proceeds, as it were, from the intellect’s likeness to the senses[46] to its distinction and difference from them.

St. Thomas and Aristotle are certainly admirable for their brilliant discovery and articulation[47] of this argument for the human soul’s subsistence. But more than them we should admire the human mind itself, since this way it has of coming to know itself is no human innovation. The fact that such a path lies open to it is not due to anyone’s philosophical genius or imagination. No, it is inscribed in the very nature of the human intellect. Its true origin, therefore, is no philosopher or any other human being, but is human nature’s author Himself.

We should be moved with joy and gratitude at the thought. How wonderful, how provident, that something as desirable to know as the subsistence of the human soul and the incorporeality of the human mind can be known by so decisive and perfect an instrument as a demonstration propter quid.[48] How deplorable, then, that the argument is so rarely considered, and is received by too many of those few philosophers who do consider it as something less than it really is—as a probable argument, or worse, as a piece of sophistry or pre-scientific, medieval bungling. In truth, it is one of the greatest common goods of reason. But access to that good must diminish considerably as long as published criticisms of it are left unanswered. So let those who see it for what it is profit from it themselves. But let them also, in their spare moments, remove whatever obstacles to it they can that lie in the way for others.

 

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