In Summa Contra Gentiles IV.39 Thomas Aquinas summarises the teaching of the catholic faith on the incarnation of the Word as follows: “And thus, even as in the Trinity there are several persons subsisting in one nature, so in the mystery of the Incarnation is there one person subsisting in two natures.” This is a very helpful summary: one nature and three persons in the Trinity, two natures and one person in the incarnation. But what does it actually mean? While Thomas will take the remaining chapters of the section to unpack it, we struggle to follow him because the notions of ‘nature,’ ‘person,’ and ‘subsisting’ as Thomas used them have become alien to us. What do they mean?

‘Nature’ – This term is equivalent to ‘essence’ and sometimes ‘substance’. It means, roughly speaking, that which makes something what it is. Think about a knife and a fork on a table. Suppose that both weigh the same, both are made of the same kind of steel, both contain the same number of atoms. What makes one a knife and the other a fork? Well, one piece of steel is the form of a knife, the other is in the form of a fork. However, the fork isn’t the only fork in the world, there are millions of forks across millions of tables. Some of them are plastic, some of them are steel, some are large and others small. What makes them all forks? Well, that they share the nature of a fork. In this sense ‘nature’ or ‘essence’ is that which determines what something is.

‘Subsistence’ and ‘Supposit’ – What does Thomas means when he speaks of three persons ‘subsisting’? What is subsisting and how does one go about doing it? Subsisting is being an existent individual thing and this differentiates it from a nature or an essence. How? Well, this time, think about the word ‘dog’. When you read the word ‘dog’, something came into your mind but it probably wasn’t the dictionary definition of the word ‘dog’, but a picture of an actual dog; let’s call him Fido. This is because the essence of ‘Dogginess’ doesn’t actually exist: you can’t touch it or feel it, and it will never get your hands wet with its slobber. No, what barks and wags and pants is a subsistence of ‘dogginess’, an individual existent dog called Fido. Fido is a subsistence, sometimes called a hypostasis in this kind of literature, of the nature or essence ‘dogginess’. This is true of all natures and essences, mugs have a ‘mug-nature’ but ‘mug-nature’ only exists in individual subsistences which you can point to and say ‘that is a mug.’ A subsistence is the individual existent instantiation of any particular essence, whether that be a mug, a fork, a dog or a human. That means that if you’re reading this, congratulations, you are subsisting. This helps us, then, understand what a ‘supposit’ is. A supposit is something that subsists. The mug on my desk is a supposit of mugness, Fido is a supposit of dogginess and you are a supposit of humanity. Sometimes the words ‘hypostasis’ and ‘substance’ are used to mean the same thing.

‘Person’ – But Thomas doesn’t use the word ‘subsistent’ or ‘supposit’ as a noun in the above definition. He speaks of ‘three persons subsisting’ and ‘one person subsisting.’ What does the word ‘person’ add? Well, Thomas affirms the ancient definition of a person, coined by the philosopher Boethius, that a person is an ‘individual substance of a rational nature’. What does that mean? Well, ‘individual substance’ basically means what we just discussed under the term ‘subsistence’. Somewhat confusingly, ‘substance’ can sometimes mean a ‘nature’ or sometime a ‘subsistence’ and here it means the latter. So, a person is a ‘subsistence’, an individuated existent thing of a particular nature. But what makes something a person isn’t just subsisting: even a mug can do that. What marks something out as a person is having a ‘rational nature’. By this, Thomas means anything that possesses an intellect and will, and with them the ability to be self-directing. Persons are not merely acted upon or made to act but act ‘of themselves’. Thus, of all material beings, only human beings are persons and, contra some modern ethicists, all human beings are persons.

To put it all together: imagine that Fido is going for a walk with his owner Bill. Fido is a subsistence of dog nature and Bill is a subsistence of human nature. But Bill is a person while Fido is not and this is why it is Bill that will determine the route of the walk and not Fido. Bill, as a human being, has a rational soul that is able to freely make choices, and is not driven solely by sensation and instinct, like Fido.

Since both the divine nature and human nature are ‘rational’, subsistences of each are persons. However, as soon as we say that, we understand why both the Trinity and the incarnation are mysteries. In the Trinity, three subsistences, three persons, share the same numerical essence, in a way that no three human persons do, (and are thus one God) while each being individuated from each other. Indeed, in God there is no distinction between his essence and his subsistence, since He is an infinite and simple being and therefore subsistent being itself, but we don’t need to get into that here. In the incarnation, one person, the Word, subsists in two natures, divine and human, without therefore being two persons. Clearly these are both truths beyond our capacity to comprehend and unlike what we usually come across in our everyday lives, where each subsistence has one nature and nature subsist in many separate supposits. However, understanding what the terms, ‘person’, ‘subsistence’, ‘supposit’ and ‘nature’ mean in their ordinary usage will help us apply them to these divine mysteries so that we stick to the truth as revealed to us in the Scriptures.

Graham Shearer is a PhD candidate at Union Theological College in Belfast, where he lives with his wife and children.You can follow him on Twitter @GJShearer.

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