Friday, 16 August 2013

Question 76 – The Union of the Soul with the Body

Why this Question Matters

Aquinas defined the soul to be the first principle of life in living things and devoted Ia.q75 to considering the soul in itself. Now it is time for him to turn to a consideration of how the soul and the body are united. The answer to this question is that the soul is the substantial form of the body; in a way the demonstration of this is simply a matter of definition within the metaphysical system that he has chosen. The challenge in this question is to show that such an approach makes for a coherent whole and that alternative approaches do not. At the time Aquinas was writing and for a period of years after his death, these sorts of questions were highly controversial. In time, Aquinas’s view was seen to be the correct one and later ecumenical councils of the church identified the approach that he took with part of the deposit of faith of the church.

The Thread of the Argument

A1: We’ve already seen that Aquinas claims that the soul is the form of the body; here he turns to a formal consideration of this question. Aquinas’s answer reiterates what he has already said: nothing acts except insofar as it is actually such-and-such, and that by which it is actually such-and such is precisely its form. What distinguishes living things from the inanimate are their vital operations; the soul is the ultimate internal principle of those vital operations. Once one has taken on-board those fundamental compositions of matter/form, potentiality/actuality and being/essence that lie at the heart of Aquinas’s metaphysics, the idea that the soul is the form of the body really becomes a matter of definition: the body must have a form that explains its fundamental operations; the soul is just what we call that form. The real content of an enquiry into the soul becomes an enquiry into what the soul is and does as a form.

Although Aquinas’s direct answer to the question posed by the article is quite short, the article itself is rather long! The reason for this is that Aquinas takes this opportunity to address the faults of a number of alternative theories about the relationship between the soul and the body. He does this in the remainder of the body of his answer and in his replies to the objections.

In pursuing this ambition, Aquinas starts off by inferring that which someone who claims that the soul is not the form of the body would have to concede about the relationship between soul and body. To do this, he makes a turn to the subject of intellectual understanding: intellectual understanding is not just something that happens out there, it’s something that belongs to each individual human. Socrates experiences that it is he himself who understands. There are three possibilities for the way in which intellection can be attributed to an individual human: the whole person can be doing the intellection (ruled out by Ia.q74.a4); per accidens (which is clearly nonsense); and through a part of the human being. This latter is the only feasible option, so if we grant that the soul is the principle of intellectual understanding in each individual, then we must allow that intellection is attributed to a part of that person and hence the soul is part of the person. The question then becomes how that part is united to the body.

Aquinas attributes to Averroes (Ibn Rushd, the Spanish Muslim philosopher known as “the Commentator” in the middle ages) the idea that the union between soul and body is effected by the intelligible species. We recall that these are what the active part of the intellect abstracts from the phantasms of the imagination for impression onto the passive part of the intellect. The idea is that phantasms are inherently bodily (produced by the sense organs), the passive part of the intellect is part of the soul and therefore the intellectual species act as the “glue” between the body and the soul. Aquinas finds fault with this, claiming that this explanation is not sufficient to account for the subjective nature of intellection: it simply doesn’t explain Socrates’s intellection as an action of the person of Socrates. Aquinas makes the analogy with sensation: the intelligible species exist in the passive intellect as visual sensations exist in the visual power. But the existence of visual sensations in the visual power does not explain the subjective experience of seeing; likewise the existence of the intelligible species in the passive intellect does not explain the experience of intellection of those species. As Aquinas puts it, the existence of the intelligible species in the passive intellect explains that the corresponding phantasms are understood, but not that Socrates understands.

Aquinas then turns to the opinion that the soul is united to the body as a mover in such a way that the action of the intellect can be attributed to the union. He finds several faults with this point of view. For example, the intellect does move the body, but it moves the body through an appetite which itself presupposes the action of the intellect. Socrates does not understand because he is moved by the intellect; rather he is moved because he understands. Similarly, Aquinas argues that attribution of the action of the intellect to the union of a part with the rest implies that the union cannot be considered as a unitary whole.

The objections to this article all hinge on a too narrow understanding of form and of how a form may enter into composition with matter. If we think of a natural object like a rock or of an artefact like a house, then when such objects go out of existence, their forms go out of existence as well. But the human soul is subsistent; it does not go out of existence when the body dies. Natural forms give a determinate existence to their matter/form composite, but the human intellect is able to grasp the quiddity of all things, which is something thoroughly indeterminate. For a natural object a form is what gives it esse; it is that by which the object exists. So it seems that esse does not belong to the form itself. These objections are fundamentally to do with what a form can be; in particular, the human soul is both subsistent and communicates the ability to the body to be in a state of indeterminacy as to the actual objects of cognition. Aquinas’s answers to these objections amount to pretty much the same thing: the fact of the existence of the intellect in human beings and the fact that this must have an ultimate internal explanation show that forms can indeed have these sorts of properties. Perhaps Aquinas’s task here has been made easier by the fact that he has already considered the angels; although the existence of the angels is a matter of faith, the fact that their existence is amenable to metaphysical explanation provides the materials to show that the spiritual human soul is also amenable to such explanation.

A2: A powerful idea at the time of Aquinas, and one which went on to have much influence after his death, was the idea of monopsychism; that humans share in one single intellectual soul. A component of what was known as Averroism, it provides a simple explanation for the fact that separate humans appear to have common perception and cognition of external objects. Indeed, this is the basis for the several of the objections supporting monopsychism. In particular, the third objection observes that if there are distinct human intellects, then the intelligible species of an object received in different intellects will themselves be individuals. But what is received in the intellect is a universal; so this appears to be a contradiction. The fourth objection takes this further: these individual intelligible species in the intellects of two separate individuals will have to undergo some process of abstraction in order to arrive as what is common to the two. But this seems to make the intellect of an individual something that is akin to the imagination; that is, something from which abstraction takes place. Again, this seems to be a contradiction, in this case to the nature of the intellect itself. Finally, the fifth objection observes that the understanding in the mind of a student is the same thing as is in the mind of the teacher; if it is numerically equal, then it must be present in something shared by them.

The first objection returns to the question left dangling at the end of the previous question: as we saw in the case of the angels, immaterial substances are not individuated by matter; therefore if they are to differ from one another, they must differ in species.

Aquinas’s response is thoroughly uncompromising: “it is altogether impossible for there to be a single intellect for all men”, “it is altogether impossible and absurd to posit a single intellect for all men”. Clearly this was an important issue, as he takes the trouble to provide a number of answers. The first answer assumes Platonism, the second an Aristotelian framework and the third purports to make no assumptions about how the soul is united to the body!

From a Platonic point of view, identifying a man with his intellect, the answer is clear; the existence of only one intellect would imply that the distinction between two individuals would not be of their essences, an absurd position. Likewise looked at from the position of an Aristotelian, numerically diverse things cannot share in one form.

To attack the problem more generally, Aquinas considers how the intellects of this man may be united with the intellect of that man. They might be united as a single principle agent with two instruments; or as a two different principle agents with a single instrument; or as a single principle agent with a single instrument. Whichever way you look at it, the inevitable consequence is that if there is only a single intellect then there can only be a single knower. We end up with a situation where there can only be a single knower and a single act of understanding. We can be quite happy with the idea that there can be different phantasms of, say, a rock in our hypothetical single intellect and that these could be the phantasms of different people; but the form in the potential intellect is an intelligible species abstracted from the phantasms from which a single universal concept is abstracted. Each person would have the same single universal and there would only be a single act of understanding. This means that the act of understanding could not distinguish this man from that man.

In reply to the third and the fourth objection, Aquinas observes that it is not the individuality of an intellectual soul that would provide a barrier to understanding universals, but rather any materiality associated with the soul. It is the immateriality of the soul that allows for the complete abstraction from material conditions. He also points out that even if there were only one human soul in the universe, it would itself be an individual and the intelligible species through which it derived understanding would itself be an individual. Furthermore, what is understood in the intellect exists in the intellect as an image of the object of perception; however, what is understood by the human is the object of perception itself. It’s perfectly possible for the same thing to be known by different knowers, as what they know is the same thing.

The trap left over from Ia.q75.a7 that is raised in the first objection is dealt with by observing that although the intellective soul has no matter in its make up to individuate it, it still is the form of a certain parcel of matter; it is this latter that serves to individuate it. Lest one should worry that this individuation goes away when the body dies, the reply to the second objection observes that the soul retains its own being (it is, after all, subsistent). It has existed as diverse from other souls; in retaining its being it remains diverse.

A3: We’ve seen that it is the intellective soul that makes a human being to be a human being; but are there separate forms that makes the human to be alive in the first place and then to be sentient? Are the three forms, the nutritive, the sensitive and the intellective “layered” on top of one another in the human being?

Once one has accepted that the human soul is the substantial form of the body, argues Aquinas, then this position really makes no sense. Things do only have one substantial form, the form that makes them to be what they are. In the first place, denying the unity of the substantial form would be to deny the unity of the thing itself; a collection of substantial forms would lead to a collection of beings rather than one unified whole. In the second place, the relationship between substantial forms would either have to be accidental or per se. If it were accidental then we would be saying either that an animal was accidentally a human or that a human was only accidentally an animal, neither of which makes any sense. If it were per se, then we would have to be looking for the definition of each one in the other; and one cannot find the definition of human in the definition of animal. Finally, the operations of the soul work as a unity, sentient and intellective, they do not impede each other as they might if they arose from two separate substantial forms.

The third objection raises the important issue of human development. In his answer, Aquinas acknowledges that in the development of the embryo it would seem that there is first a sentient soul that is later replaced by an intellective soul. He will return to this issue in more detail latter (Ia.q118.a2). In brief, the reasoning behind this lies in the fact that a form can only be received in matter that is fitted for the reception of that form. The science of the day suggested that the matter of the early embryo was not suitable for the reception of an intellective soul and hence ensoulment was thought to happen at some time after physical conception. In is interesting to speculate that Aquinas would probably be of a different opinion in the light of modern embryology.

In his reply to the fourth objection, Aquinas makes an important point about how we perceive and categorize the world. The substance of the objection is that when we think about the genus animal and the difference rational that characterizes humans there would seem to be a correspondence between matter that determines the genus and form that characterizes the difference. This suggests that a body animated by a sentient form should be considered as the “matter” that is informed by the soul. Aquinas answers that, in the world, there is no need for a real diversity corresponding to how we understand the world: “reason can apprehend one and the same entity in diverse ways”. The idea of a genus is one of our conceptual ways of organising our thinking about the world: we abstract the idea of the genus animal from our observations of the commonality between humans and other animals and arrive at the difference rational from the same observations. This genus/difference classification is how our mind thinks about the world, but it need not correspond exactly to the actual way that the world is put together.

A4: At first sight, the existence of this article may seem a puzzle; hasn’t Aquinas already answered the question of whether there are other substantial forms in man than the soul? Indeed the first objection appears to be a rehash of the fourth objection of the previous article. The point is that in the third article Aquinas was dealing with the specific idea of nutritive, sentient and intellective substantial forms as the formal structure of humans. Here, as the sed contra suggests, he is dealing with full generality: a being has a single substantial being; the substantial form of a being is what gives being to the being; the soul is the substantial form of the human; end of argument. The body of the article is really just a swift reiteration of what substantial and accidental forms are and how they differ; a revision exercise for those students who may have nodded off earlier in the course at a vital moment. A substantial form is what gives being absolutely to something; an accidental form gives being in-a-certain-sort-of-way to something. The intellective soul is the substantial form of man and the fact that a human being is also an animal and a living thing merely reflects that the nutritive and sentient elements of humans are subsumed under the intellective soul. Aquinas uses the (slightly misleading) terminology that the intellective soul virtually contains the nutritive and sentient souls. This doesn’t mean that the latter are actually present as forms in the intellective soul; rather, the intellective soul includes everything that would be contained in the nutritive and sentient souls.

The fourth objection gives Aquinas the opportunity to talk about mixtures: it would seem more reasonable to consider that the basic elements that make up the human body maintain their substantial existence, therefore each element in the body has its own substantial form and the body is really a mixture of these elements. Against Averroes and Avicenna, Aquinas argues that in a mixture there is not really something new created but rather that if one looks close enough one will see the individual elements in place. The substantial forms of these individual elements in the mixture do not remain actually in existence but exist virtually in the substantial form of the whole mixture. The substantial form of a whole “takes over” everything to do with the substantial forms of the parts that go to make up the whole.

A5: Given that the intellective soul is such a remarkable thing, it doesn’t really seem fitting that it is united with such a pathetic body. On the one hand the body is incorruptible whereas the soul is not, on the other hand the human body, compared to animal bodies, seems rather lacking in physical attributes.

Aquinas answers that because matter exists for the sake of form, rather than form for the sake of matter, the reason matter is the way it is entirely due to the form itself. The intellective human soul, on the bottom rung of intellective substances, has to possess the power of sensing as it does not have the sort of immediate access to knowledge that the higher intellective substances do. Therefore the soul has to be united to the sort of body that is suitable for this purpose. The human body, Aquinas claims, is the most balanced with regard to its sensory equipment amongst the animal kingdom. Would this claim stand up to modern scrutiny? The human body certainly does not have the most acute of senses in the animal kingdom, but Aquinas might respond that this is not the point: what matters is how they work together such that the common sense (the internal sense that integrates the input of the external senses) can make a better model of the world for the intellect to ponder upon than can other animals.

The reply to the fourth objection (that other animals have much better built in tools and defences) is interesting: the intellective soul is ordered towards the comprehension of everything, therefore giving it determinate natural defences or weaponry would be futile. What humans are equipped with is the means by which they can fashion what they need for their particular circumstances. The hands, for example, are the “instrument of instruments”.

A6: When a new thing comes into existence a new substantial form replaces the form(s) out of which the new thing is made. It’s a fundamental principle that the matter out of which a new thing is made must be suitable to the reception of the new substantial form. But surely this means that there must be a number of accidents present (such as size and shape) that make the matter suitable for the reception of the new substantial form and therefore the union of substantial form to matter (and therefore of intellective soul to body) must be mediated by some accidents? It seems that these accidents are prior to the substantial form in the new thing.

This confusion arises from failing to understand how certain things exist virtually in a substantial form. The matter out of which something new is made does have to have certain accidental properties that allow for the new substantial form to take its place. But these accidents are also contained virtually in the new substantial form, so that when the new substantial form is in place they are proper accidents contained virtually in the substantial form rather than being added to or mediating the substantial form. The substantial form takes over, as it were, what was formally attributed to the accidents.

A7: The sixth article asked whether the union of the soul and the body is mediated by accidents. This article poses the further question: if not mediated by accidents, could the union be mediated by some body or bodies? The answer is straightforward, being simply a reiteration of what has gone before: the union of a substantial form with matter is not mediated by anything; to think so is simply to misunderstand how it all works.

That Aquinas makes the point about the union of a substantial form with matter over and over again shows that he was either dealing with some rather dense students, which is unlikely, or that the issue was a live one at his time. There were a number of opinions on the union of soul and body around at the time and much heat was generated in their discussion. Ultimately, Aquinas’s opinion won the day; mostly because it was difficult to fit the other theories into any sort of systematic metaphysics. Indeed these alternative ideas tended to last longest amongst those who were not particularly systematically minded!

A8: If one, by accident I hope, cuts an earthworm in two then often the two halves continue to thrive. This suggests that the form of the earthworm, its sentient soul, is not strongly localised to any part of the body. Is the same true of the intellective soul of the human? After all, cutting a human in two doesn’t usually end as well as it does for the earthworm.

The soul is the substantial form of the body; it is the thing that makes the being what it is. So, in particular, the soul must inform all parts of the body, making them to be what they are and making them to work together as a whole. So, the soul must be present in the whole of the body and in all its parts. What is less clear, perhaps, is whether the soul as a whole must be present in all parts of the body.

In order to answer this more specific question, Aquinas asks us to consider what we mean by wholeness and its relation to parts. He claims that there are three types of wholeness and related division into parts: one can divide a material object into quantitative parts; one can divide something into rational and essential parts, like the parts of a definition or the form and matter of a whole; one can divide a power into its virtual parts.

If we think about a patch of surface that is coloured white, then we observe that if the surface is broken up then each bit of surface will still be white; whiteness is both in the surface as a whole and in each of the parts individually. But this first quantitative type of wholeness in the parts only happens in certain circumstances where the whiteness is uniformly spread about the surface. For something like a soul that is informing different types of parts it simply cannot happen; there is a different relation to the whole and to the parts. On the other hand, the second and third types of wholeness can be applied to substantial forms. The soul does exist as a whole in each part of the body from the point of view of its essence but it does not exist in each part of the body with respect to its powers. For example, it is in the eye with respect to seeing and in the ear with respect to hearing but not vice versa. What is more, it does not exist in the whole body in the same way that it exists in the parts: it is primarily in the whole and secondarily in the parts insofar as they are ordered to the whole.

Handy Concepts

  • The definition of the Council of Vienne, that the soul is the form of the body, was reiterated as a definition of the faith in the eighth session of the fifth Lateran Council (1512-17). One might infer from this that there had been some resistance to the idea! One should note that the Lateran definition went further, confirming the teaching of the second article.
  • The fifth objection to the first article and its answer might be seen as a faint parallel to the question of whether there is one act of being in the hypostatic union of the God-man Jesus Christ.
  • The fifth objection to the second article should be dedicated to teachers the world over.
  • The seventh article makes mention of quintessence (or fifth essence). This was considered to be the form of matter that filled the universe above the terrestrial sphere. Funnily enough, the term has returned to modern physics attached to a hypothetical form of dark energy.


  • The process of intellection abstracts a universal of the object of cognition from the phantasms present in the imagination. For each individual this results in an individual act of understanding, that is, each individual has a universal present as the expressed species corresponding to the intelligible species. If the object of perception is the same for two individuals, in what way do they share the universal that they abstract from their phantasms? In what way are the universals of “rock” different in each individual?
  • The doctrine of the unity of substantial form (article 3) was highly contested during the Middle Ages. The doctrine was even condemned by two archbishops of Canterbury, Kilwardby and Peckham.
  • A strict materialist might argue that human animals are accidentally human (article 3).
  • At what point in the formation of a mixture (objection 4 to article 4) do the forms of the elements disappear and the form of the mixture appear?