Having considered God and then spiritual creatures, Aquinas now turns to the consideration of human beings. He is going to spend a lot of time considering humanity (the whole of the second part of the summa is about human beings) before turning to the God-man Jesus Christ in part three. For the moment, we will be looking at the so-called Treatise on Human Nature that stretches from question 75 to question 102 of the first part of the summa. In his introduction to question 75, Aquinas tells us that he will consider the nature of humanity first (in questions 75-89) and then the production of humans (in questions 90-102). In thinking about the nature of human beings, he will consider the nature of the soul (questions 75-76), the powers or capacities of the soul (questions 77-83) and the operations of the soul (questions 84-89).
It’s always important to try not to interpret Aquinas in terms that apply to later philosophical ideas; this is especially true in the field of philosophical psychology covered by this treatise. For example, although Aquinas identifies the soul as a subsistent form, he is not a substance dualist in the sense of Descartes. Aquinas believes that the soul is the form of the body and that although the soul is subsistent, one should not interpret the soul and the body as being substances with an independent existence. Likewise it’s important to note that Aquinas uses terms such as intellect, mind, imagination, appetite with precise meanings that slot into his particular psychology. One should not read into his thinking elements derived from modern use of similar terminology.
Why this Question Matters.
In this question, Aquinas is going to discuss the nature of the soul considered in itself, turning to the way in which the soul is united to the body in the next question. Aquinas is aiming for the position that, in living things, the soul is the form of the body. Moreover, in human beings, the intellective nature of the soul implies that it is immaterial, subsistent and incorruptible. However, as is so often the case in the summa, he is going to work up to that position incrementally. He first considers whether the soul is a material body and then turns to the difference between the souls of human beings and non-human animals. The former possess an intellective soul, whereas the latter do not. This has the extraordinary consequence that the human soul is subsistent and, in fact, is a subsistent form. As a consequence of this, the human soul is incorruptible. Even while describing the extraordinary nature of the human soul, he is careful to emphasize the bodily nature of the human being as well; the soul is not the human person, only the soul/body composite is that.
All this, of course, tallies well with Christian doctrine concerning the soul; but Aquinas’s approach here is philosophical and not directly theological, firmly based on the teaching of Aristotle’s de anima.
The Thread of the Argument
A1: Aquinas starts his enquiry into the nature of the soul by asking whether the soul is a body. If we think in terms of the modern philosophy of mind, what he is considering here is part of the argument of whether the soul has a material explanation; can we, even if only in principle, reduce the explanation of the soul to an explanation of the behaviours of some material substances? In particular for this question, can we identify the soul itself with some material body? In order to answer this question Aquinas is careful to give a definition of the soul: it is “the first principle of life in those things around us that are alive”. In other words, a soul is the type of thing that explains the difference between something that is animated and something that is inanimate. It’s worth noting that Aquinas is using the term principle as a very general description of that part of a subject which explains something about the subject. He is looking for something about the subject itself which explains its mode of being.
In answer to those who would argue that the soul is a material body, Aquinas’ argument is disarmingly simple. We can see that living things can do certain things that non-living things cannot; we call these vital operations. It is clear that some vital operations at least can have principles that are material: a principle of seeing is the eye, for example. But what Aquinas is concerned with here is the first principle of these vital operations; being such a first principle of life or being alive cannot belong to a body simply by reason of it being a body. If the latter were the case, then every body would be a soul or every body would be alive. Therefore, there must be something added to a material body that is alive that makes it to be alive; it is a body of a certain sort and the soul is what distinguishes it from inanimate bodies.
The second objection raises a subject that is going to be important in the sequel, that of cognition. The objection itself doesn’t seem to be all that coherent, but argues that the soul must be a body if it is to be able to have cognition of material objects. The objection appears to be coming from the point of view that in order to have cognition of material objects, some likeness of these objects must exist in actuality in some way in the body; like must be like in order for cognition to occur and therefore such likenesses must occur in the nature of the body. Aquinas argues that the cognition of something involves the body in passing from potentiality to actuality; the body is in potentiality to form a likeness of a material object. But this does not mean that the body need have the likeness of the material object in its nature, only that the potentiality to form such likenesses exists in its nature. In the sequel, Aquinas will say much more about how the body passes from potentiality to actuality in the process of cognition.
A2: Just as the first article of this question may disappoint for its limited scope, so this second article may take the breath away for its audacity. The human soul is subsistent; that is, it has some sort of existence apart from the body.
That the human soul continues in existence after the death of the body is, of course, a matter of divine revelation. But here, Aquinas is intent on arguing for part of this position on purely philosophical grounds. One immediately gets a sense of the direction of Aquinas’s argument from the first line of his response; he identifies the human soul as being the principle of the intellectual operations in humans. These intellectual operations allow us to operate on our perceptions of material objects in order to have cognition of their natures. What is more, we can have cognition of all material natures. Aquinas then argues that since this is the case, a human intellect cannot have within itself the nature of any particular material body, because such a presence would inhibit the cognition of that particular material nature. Likewise, intellectual understanding is not gained through a bodily organ, since the determinate nature of that organ would prevent us having cognition of the nature of that organ. (N.B. Aquinas does not argue that bodily organs are not involved in the total process by which we arrive at an intellectual understanding of a material nature; simply that they cannot be involved in the final intellectual cognition of such a nature.)
This means that the mind or intellect has operations (the intellectual operations) of its own that do not involve a material body. But in order to have its own operations it must be subsistent, as only those things that are actual can have operations. Therefore the intellect is both immaterial and subsistent.
The replies to the objections contain some important principles. The common teaching of the time was that the soul is the form of the body; therefore it seems to make no sense to speak of the soul as being subsistent, a this-something (hoc aliquid) since it is the form-matter composite that is the this-something, not the soul on its own. Aquinas answers that we can think of something like a hand as a subsistent thing, a this-something, that is a part of something that is a complete subsistent thing. The human soul is subsistent in the sense that the hand is subsistent as a distinguishable part of some greater whole; but it is not subsistent in the sense of being complete in itself. Likewise, when we think of how the operations of a part of the body (such the eye seeing) relate to the operations of the whole we should think in terms of the whole operating by means of the operations of the parts. The human sees by means of the eye; the human understands by means of his soul.
A3: Aquinas has argued in the second article that the human soul is immaterial and subsistent. What about non-human animals; are their souls subsistent too? Aquinas takes it as foundational that what distinguishes the human being from the non-human animal is that the former is in possession of an intellect whereas the latter is not. Non-human animals are sentient (and therefore in possession of a sentient soul) but they are not intellective; they cannot extract and abstract the quiddity of things from their sensations of things.
Aquinas acknowledges that Plato considered non-human animals to have subsistent souls, on the basis that although intellective activity is distinguishable from sentient activity, they both arise from immaterial principles. Aristotle, against Plato, argued that only intellective understanding is incorporeal and that sentient activity is completely associated with bodily change. Aquinas aligns himself with this position, arguing that sentient activity arises entirely from the activity of the conjoined body and soul. The sentient soul has no activity in and of itself and therefore cannot be considered to be subsistent. Again, it is important to note that Aquinas is quite willing to admit that bodily change occurs as part of the complete process by which humans arrive at an intellectual understanding of some object; it is the final step of intellection that is entirely immaterial in human beings.
A4: If one is a substance dualist, there is a danger that one may believe the body to be only a sort of container for what is really me; my soul. Similarly, an overemphasised spirituality will tend to downplay the role of the body in the human person. Having shown that the human soul is subsistent, one might also think that Aquinas would lean this way. But he will have none of this; I am not my soul, I am the composite of soul and body.
Aquinas offers two arguments in support of this position. For the first he looks at how one would understand the definition of human. For all natural things, the definition refers to both the form and the matter. One might refer to the definition of human in general (in which case one sees that man comprises soul and flesh and bone) or to a human (in which case one sees that this human comprises this soul and this flesh and bone). Either way, body is irreducibly part of the definition of human. For the second argument, Aquinas turns to what has been argued in the third article; the sentient operations of an animal are irreducibly performed by the soul/body composite. This much applies to humans as much as it does to non-human animals. Therefore one cannot understand the operations of a human (and therefore the human himself) without the notion of the soul/body composite.
A5: Is it possible that the soul is itself composed of matter and form? Aquinas offers two arguments against this. In the first argument he comes out and states explicitly that the soul is the form of the body. If there were some part of the soul that was material then we could simply ignore that part and take the immaterial formal part as being the form of all the rest and call that the soul.
The second argument is perhaps more interesting in the sequel and refers in particular to the human soul rather than to souls in general. The human soul is intellective and as such has the cognition of the quiddity of things. In order to have this cognition of a thing (such as a rock), the form of that thing has to exist in some manner in the soul of the person having the cognition. What is more, the cognition of that rock is a cognition of the universal, the what-ness of it, abstracted away from the particular rock. Now, we recall that matter is the principle of the individuation of things (this rock is different from that rock because it is composed of a different lump of designated matter). If the intellective soul contained matter, then the forms it receives in cognition would inform the matter of the soul and we would only receive those forms as individuals. Therefore since we know that the intellective soul has cognition of universals, it must be immaterial.
Aquinas emphasizes this latter position in the reply to the first objection, which involves a rather intricate argument about the connection between the neo-Platonic idea of procession and the Aristotelian ideas of actuality and potentiality. The soul moves from potentiality to actuality in cognition and therefore it seems that it would have to participate in pure potentiality (that is, prime matter) in order to do this. But the intellective soul does not receive individual forms in the way that prime matter receives individual forms. Rather the intellect receives absolute forms (that is, universals) and these are not individuated in matter. The receptive potentialities of prime matter and of the intellective soul differ from one another in this mode of reception.
The fourth objection raises the possibility that we might have too elevated a notion of human soul. A subsistent, immaterial form begins to sound a bit too much like God himself. Fortunately Aquinas has already gone through this problem in the case of the angels (Ia.q50.a1); in parallel with that argument, one must look to the composition of actuality and potentiality as being more primary that the composition of matter and form.
A6: One of the consequences of the human soul being a subsistent form (article 2) is that it must be incorruptible. Once having been created, the only way that a human soul ceases to exist is for God to remove its participation in being; it does not naturally die or degenerate or decompose with the death of the body.
To show this, Aquinas first distinguishes between per se and per accidens generation and corruption. When one considers the components of something composite, where the components do not subsist on their own, such as accidents or the forms of material objects, they are generated and corrupted per accidens. That is, they come into being and go out of being because the composite itself has come into or gone out of being. On the other hand, things that subsist per se can only come into being and go out of being per se. Their generation and corruption do not depend on some composite apart from the composition of actuality and potentiality; in particular, they only go out of being if their actuality, their esse, is withdrawn.
As the human soul is a subsistent form, it can only be corrupted per se, in common with any subsistent form. But being belongs to a subsistent form by its very nature; so in the natural order of things, it cannot cease to exist. The only way that a subsistent form can cease to exist is if God withdraws esse from it.
Aquinas goes on to offer two more arguments in favour of the incorruptibility of the human soul. The first covers the situation where one might take a wider definition of the soul to include some composition of matter and form (as in article 5). In this case, Aquinas argues that in order to corrupt, some form of contrary is required (one might think of a pathogenic attack within the body as being such a contrary). But although the human soul is able to form concepts of contraries, contraries themselves do not exist within the soul. Finally, Aquinas offers the heuristic argument that the intellect apprehends being in an absolute sense and that consequent upon that apprehension is a natural desire for that being. On the principle that a natural desire cannot be in vain, this would imply that the desire for being is never naturally frustrated for the human soul.
The third objection to this article argues that nothing can exist without its proper operation and that the proper operation of the human soul is intellective understanding using phantasms. But after death, there are no phantasms and therefore there can be no intellectual operations. In his reply to this objection, Aquinas grants that intellectual understanding in conjunction with phantasms is proper to the soul as it is united with the body. But he argues that when separated from the body it will have another mode of intellective understanding. He leaves the question of what this latter mode of understanding might be for Ia.q89.a1.
A7: The fourth objection to the fifth article of this question has already raised the spectre that the human soul as subsistent form might be too elevated a notion. The day was saved there by pointing out that a subsistent form can still be a composition of actuality and potentiality rather than having to be pure actuality. But we now recall that angels are pure forms that are compositions of actuality and potentiality; so are human souls simply some sort of species of angel? They are, after all, very similar to one another in nature in many respects. The particular question that Aquinas asks under this theme is whether the human soul belongs in some species of angel.
In answer, Aquinas reiterates the argument of Ia.q50.a4 where he showed that each angel inhabits a species of its own. Angels are immaterial and since matter is the principle of individuation within species there cannot be more than one individual angel within a species. The same argument extends to show that the human soul cannot inhabit a species already inhabited by an angel, as the human soul and an angel are clearly different (they have different intellectual operations, for example). Aquinas even goes so far as to extend the argument to the case where the human soul is considered to have some matter in it (as in article 5).
Notice that Aquinas seems to have laid a trap for himself. Surely the reasoning of this article means that distinct human souls must inhabit distinct species? Aquinas will address this problem in Ia.q76.a2.
- The Latin word that is translated in English as soul is anima. We can see the etymological connection to words such as animated and animal. The fundamental idea is that the soul is what distinguishes animate objects from inanimate objects.
- The reply to the first objection of the second article amounts to Aquinas’s denial of substance dualism.
- In this question, Aquinas uses a lot of terminology to which he doesn’t give strict definitions. He will go into much greater depth as he proceeds through this treatise and things should become clearer! For the moment, we’ll give some elementary introductions. Phantasms are introduced as things that the soul uses in its act of understanding. A phantasm is the image (composed of all sensory components) that the imagination calls to mind when thinking about an object. Similarly, Aquinas introduces the idea of the intellect which is that power of the soul that enables humans to have cognition of the what-ness (quiddity) of things. In the third article we see that Aquinas believes that non-human animals are sentient (in the sense of being able to sense things and to react to those sensations) but do not possess intellect; they are unable to abstract the what-ness of things from their sensations in the way that humans can. From a phantasm, the active part of the intellect (the active or agent intellect) extracts the quiddity of the object of perception. What results from this process of extraction is an intelligible species that is impressed upon the passive part of the intellect (the passive or possible intellect). From such an intelligible species, the intellect forms a concept of the object of perception.
- Not long after Aquinas died, the ecumenical Council of Vienne (1311-12) defined as a doctrine of the faith that the human soul is the form of the body. “…we reject as erroneous and contrary to the truth of the catholic faith every doctrine or proposition rashly asserting that the substance of the rational or intellectual soul is not of itself or essentially the form of the human body, or casting doubt on this matter”. (Decrees of the Ecumenical Councils, Vol 1, tr. Tanner, p 361. See also DS902).
- As non-human animals do not possess subsistent souls, they do not accompany us to heaven (or hell).
- In thinking about the fourth article, we might call to mind that Aquinas wrote, rather poetically, in his commentary on 1 Corinthians 15 (lecture 2, para. 924) “…but the soul, since it is part of man’s body, is not an entire man, and my soul is not I”.
The idea that the soul itself might be a composition of form and some type of matter (article 5) is associated with Ibn Gebirol (Avicebron, 1021-70). The idea gathered some momentum in the Franciscan school after Aquinas’s death.
- Aquinas wrote much about human psychology in Book 2 of the summa contra gentiles; in many ways this material is more systematically arranged than that of the summa theologiae. In addition, he wrote a commentary on Aristotle’s de anima and there is a collection of Aquinas’s disputed questions de anima.
- By the end of this question we know that the human soul is an immaterial, subsistent and incorruptible form; the intellective nature of the human soul is what differentiates it from the non-human animals.
- As we have seen, Aquinas takes for granted quite a lot of terminology to do with the way that the human soul works, even as he is explaining it. We will provide some background in each question sufficient to understand the question and will provide a summary of how everything fits together once we have reached the end of the treatise. For the time being, a useful but compact reference for understanding Aquinas’s approach to the human soul is Chapter 4 of Wallace’s The Elements of Philosophy. A useful reference for placing Aquinas’s approach to human psychology in a wider context is Maslin’s An Introduction to the Philosophy of Mind. One may, of course, also wish to dive into Aristotle’s de anima!
- In the first article, Aquinas’s argument that the soul is not a body appears to amount to nothing more than an emphasis of his definition; the soul is that which differentiates between living and non-living things. Aquinas takes it as self-evident that there is a fundamental qualitative difference between things that are alive and things which are not. It’s important to note that Aquinas’s ambitions in this article are very limited; he is not arguing for the immateriality of the soul (he will do that later). Rather, he is arguing that some material bodies possess vital operations and that such operations cannot be explained simply by the fact that a body is a body.
- The line of reasoning that Aquinas takes in the second article may disappoint, given all the avenues that he could have taken (such as the argument from intentionality, the argument advanced in the fifth article, or the arguments presented in the second part of the summa contra gentiles) and even given that he could have bolstered the argument that he has used. Perhaps we are seeing a consequence of the order that Aquinas has chosen to present the material on human nature; in order to present a more convincing argument he would have had to develop more sophisticated preliminaries; perhaps he felt this not suitable to the grade of student he was writing for!