Home » Metaphysics » Does Aquinas or Scotus Succeed in Proving that the Soul is Immaterial?

Does Aquinas or Scotus Succeed in Proving that the Soul is Immaterial?

Gill Prestidge
University of St Andrews


In this essay I am going to be arguing that Aquinas does not succeed in proving that the soul is immaterial, on the grounds that his argument results in two positions which cannot coexist, however both he and Scotus do give us a good reason to think that it might be, as several of their key premises are difficult to refute. Scholastic thought in the area of the philosophy of mind began with Aristotle and involves talk of souls rather than minds, but the soul encompasses an intellectual power (possessed only by humans), a sensitive and locomotive power (possessed by humans and animals) and a nutritive power (possessed by humans, animals and plants). Nowadays, we would probably say that these powers are the operations of the mind and brain, and scarcely mention the soul. However, our concern here is less the semantics, but the nature of the soul or mind and so I will use the term soul just as the scholastics and Aristotle do to avoid confusion.

Thomas Aquinas’ philosophy of mind is an attempt to reconcile the teaching of Aristotle with that of Christian doctrine, and therefore philosophy with theology; the primary concern being Aristotle’s non-reductive physicalism and the Christian teaching of the afterlife, for which St. Paul and Augustine are the authorities. Aristotle’s main point is that the soul is the form of the body; ‘form’ in the sense of the configuration of elements that something is composed of. He believed that the soul was this configuration and this left him with some form of non-reductive physicalism; non-reductive because, although the mind and body are not separable, they are not identical either, and physicalism because the soul is the brain’s and nervous system’s set of capacities and they are therefore inseparable. As all this is physical, there is no problem with assigning these capacities to organs in the body, but Aristotle decides that he cannot do this with the thinking aspect of the soul; there is no organ which it links to. He holds onto this belief due to a commitment to the idea that the configuration (the concept) of an object is in the mind, but the object itself is not.

However, this presents him with a tension and results in a form of dualism, whereby the intellect is formless and therefore not blended with the body, making it only a potentiality. He attempts to solve this problem by pertaining to the possibility of two different intellects: an active and a passive. The passive is where forms reside and is inseparable from the body, whereas the active (that by which we do our thinking) is separable; the active intellect depends upon the passive and so some of the intellect is immaterial and some is not.

It is from here that Aquinas begins his work. Christian doctrine teaches us that there is an afterlife and a resurrection of the body and Aristotle argues that the agent intellect is a part of the soul which survives death, for it is separable from the body. The problem is that Christian doctrine does not give us a reason to believe in substance dualism. We see this in St Paul’s letter to the church in Philippi:

But we are citizens of heaven, where the Lord Jesus Christ lives. And we are eagerly waiting for him to return as our Saviour. He will take these weak mortal bodies of ours and change them into glorious bodies like his own, using the same mighty power that he will use to conquer everything, everywhere.

The problem for Aquinas is that his Christian beliefs lead him towards favouring physicalism, but he also supports Aristotle who takes something of a substance dualist line; he then has to find a way of linking Christian doctrine to dualism if he is to hold onto both. As it is personal immortality that Aquinas wishes to account for, it makes sense for him to argue that the agent intellect (which we saw in Aristotle) does survive death. This agent intellect is not the human person though, for it is incorporeal and immortal, so he has not yet established personal immortality, but has cleared the way for some kind of dualism.

In Summa Theologiae, 75 ad 2, Aquinas sets out to prove that the human soul is incorporeal and subsistent, amongst other things. This is our concern as it brings out his conclusion that the soul is immaterial also. The argument is in seven key parts, but this is not where the greatest problem is, as we shall see. Aquinas writes:

answer that, it must necessarily be allowed that the principle of intellectual operation, which we call the soul of man, is a principle both incorporeal and subsistent. For it is clear that by means of the intellect man can know all corporeal things. 

This final sentence demonstrates the first of the key points; that we understand that which our senses perceive through the intellect. That which receives this information cannot itself have anything it receives information about as part of its nature and, as all bodies have their own nature, it cannot be a body either. This covers points one to three. Four states that because of two and three, it cannot be supported by an organ and therefore, five – it has an operation which the body does not. This operation is in the intellectual principle (to use Aquinas’ language) and so it must subsist; that is, be dependent on nothing and stand alone. Aquinas now concludes seven, that the intellectual principle is both incorporeal (as shown by three) and subsistent (which we see in five and six). Subsistence for Aristotle is essentially the same as being a substance. This is not in the scientific sense of the word, but much more generally – a thing, an entity. What this means for Aquinas is that his argument is becoming dualist in nature with reference to the soul. This view of subsistence applies to a notion of strong subsistence, but Aquinas also speaks of a weak one, whereby something is a part of a substance which is itself subsistent in the stronger sense of the word. An example of a weakly subsisting thing could be a hand or foot; whilst it is a thing in and of itself, its existence is dependent on the body.

So what is the dilemma? For the soul to have its own operation, it must have a particular mode of existence; that is, as an individual or as part of an individual one. This seems to make sense as it would be nonsense to say that a property, such as colour or shape, is an object’s operation; an operation can only be an action. Aquinas now makes the leap from the soul’s having its own operation to its existing independently – this is strong subsistence. However, Aquinas then says:

Since the human soul is a part of human nature, it can be called this particular thing in the first sense, as being something subsistent; but not in the second, for in this sense the composite of the body and soul is said to be this particular thing.

We now see that he is introducing weak subsistence and attempting to argue that the soul is both weakly and strongly subsistent simultaneously. Before proceeding to the problems this presents, it is important to see why Aquinas would want to argue this. We have seen that for the soul to have its own operation it must be subsistent and this subsistence must be in the stronger sense because it alone is ‘this particular thing’. Aquinas now writes that in the second sense, that is, where things that are parts of a subsisting thing are excluded, it is not the soul that is ‘this particular thing’, but the composition of body and soul. It is this composition that he wishes to hold onto which results in the need for a weakly subsisting soul. As soon as the soul is given the label of ‘this particular thing’ – the ‘substance’ for Aristotle, the human body becomes superfluous and the unity of the human being – the encompassment of spiritual and corporeal substances – is lost. Both Aquinas and Aristotle stand for the belief that one can think of and understand an object because the object’s form resides in the intellect, but preserving the unity of the human being is inconsistent with the soul having an operation which the body does not share and this threatens the original idea that the soul is incorporeal. Aquinas endeavours to give an account of the soul as a strongly and weakly subsisting thing at the same time, which is impossible, but a loss of either would undermine his whole thesis and Aristotle’s too.

It is clear that this attempt will not work unless something is given up, but Aquinas does not offer us such a solution. John Duns Scotus, on the other hand, is not convinced by Aquinas’ argument, but does seem inclined to reach a similar conclusion. Scotus, as a fellow scholastic, is determined to prove the existence of an immaterial soul, but without making any claims devoid of reason. In chapter six of Opus Oxoniense, he states that three propositions would have to be established before we could reach the conclusion that the soul is immaterial through reason. These are (I) the intellective soul is the specific form of man, (II) the intellective soul is incorruptible and (III) the specific form of man will not remain forever outside the composite. It is the first of these propositions that concerns us here, but the third I wish to come back to, as its discussion shows Scotus’ own feelings on the matter, and Scotus himself suggests that the second cannot be proven even though there might be good reasons for it. It seems that we do know the first proposition by natural reason, but this would not fit with Aquinas’ beliefs about subsistence and the incorporeal nature of the soul. Scotus is not concerned about this, in fact he aims to show the difficulties in Aquinas’ argument and rework it at the same time.

There are several sections to Scotus’ main argument for the first proposition and it presents him with two conclusions. The first premise is that all sense knowledge is sense experience of particular things and not concepts. He then adds that human beings possess knowledge that is beyond knowledge of particulars; we have knowledge of concepts, deductions and inferences also. From here Scotus takes two paths: one to reach the conclusion that this knowledge we have over and above that of particulars is our specific form, on the grounds that this knowledge is understanding and it is what distinguishes us from the animals, and the other to show that the part of the soul which understands must be immaterial. It the second proof which we are interested in and from here, Scotus demonstrates that the knowledge we call understanding is something immaterial, but no organic knowledge can be immaterial because it is only sensed if it is received. This means that no bodily organ can be responsible for said knowledge and the part of the soul which does the receiving of this knowledge must itself be immaterial.

Scotus’ conclusion seems to follow on from his premises and create a coherent and valid argument, but how are we to take ‘immaterial’ in this sense? Scotus writes:

This word “immaterial” is frequently used by the Philosopher in this connexion, but it appears to be ambiguous. There are three relevant ways in which it can be understood. 

These three ways he goes onto explain are: (I) not linked to a bodily organ, (II) not extended and (III) abstract. He explains that the argument only works with the use of the first or second sense of immaterial, but the only one we can have surety over is the third. We need to get from this abstract notion of the immateriality of the soul to an incorporeal one. One way to do this would be to question Aristotle’s mind/world identity thesis and distinguish between the concept and its content. Both Aristotle and Aquinas assume that a concept can be inferred from its content, but it could be the case that the concept (or the intellect using it) is an accident of the brain and therefore material, but with abstract content.

However, even if this move does work there is still the difficulty mentioned above with the third of Scotus’ propositions; that is, that the specific form of man will not remain forever outside the composite. This problem is in fact one of Scotus’ own. Although he considers his conclusion to be likely, he does not think that they were absolutely conclusive; it is possible that the soul cannot exist apart from the unity of spirit and body. Scotus concludes the section on a priori proof with: ‘the conclusion, then, which follows from these three propositions is not sufficiently known a priori by natural reason.’

In conclusion, Aquinas does not succeed in proving that the soul is immaterial on the grounds that he requires the soul to have both strong and weak subsistence and the two cannot coexist. It seems too that Scotus’ criticism of Aquinas is fair as it is his almost unwavering acceptance of Aristotle which leads him to such a difficult position to maintain as Aristotle’s conclusions are just as problematic. Scotus shows us that progress can be made when the original mind/world identity thesis is abandoned, but that there is still a long way to go if pure reason is to secure such conclusions. We can see that a proof of this argument has not been achieved, but that both Aquinas and Scotus have given us reasons to think that an argument for the soul or mind as an immaterial entity should not be abandoned, but constructed differently with reference to the location of concepts.


Works Cited

Aquinas, T. (1265-1274), Summa Theologica. Translated by Fathers of the English Dominican Province (1947) [online] retrieved 25th May from: http://www.ccel.org/a/aquinas/summa/FP/FP075.html#FPQ75OUTP1

Aristotle (ca. 350 BC), De Anima. Translated by Smith, J. [online] retrieved 23rd May from: http://psychclassics.yorku.ca/Aristotle/De-anima/de-anima2.htm

Holy Bible, New Living Translation, (Published in 1996), United States: Tyndale House Publishers

Scotus, J., Philosophical Writings. Translated by Wolter, A. (1987), Cambridge: Hackett Publishing Company


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