What is the soul, for Aquinas? (ST I, q. 75, a. 2)
Summa article explained, no. 3. A paraphrase and reading guide, not a translation.
The short answer. For Aquinas the human soul is the first principle of life in a human body and the form of that body. It is also incorporeal and "subsistent": because we can understand all kinds of bodies, the power by which we understand cannot itself be a body or work through a bodily organ. So the soul survives death, though a human being is soul and body together.
Where this sits in the Summa
The treatise on man begins at the First Part, question 75. Question 75 asks what the soul is in itself. Question 76 asks how it is united to the body. Read them as a pair. Article 2 of question 75 is the hinge: it is where Aquinas argues that the human soul is not just a pattern of the body but something that has an act of its own.
The article just before it (q. 75, a. 1) sets the ground. The soul is the first principle of life in living things. It is not a body, because if being a body were enough to be alive, every body would be alive. The soul is the "act" of a body, what makes this matter a living thing.
The objections
Aquinas states three, and they are good ones.
- "This particular thing." A subsisting thing is a "this". But we call the composite of soul and body "this man", not the soul. So the soul does not subsist.
- The soul does not act. Aristotle says that to say the soul understands is like saying the soul weaves or builds. If the soul does not act, it does not subsist.
- No act without the body. Even understanding needs images drawn from the senses (Aquinas calls them phantasms), and those need the body. So the soul has no operation of its own.
Sed contra
Augustine (De Trinitate x, 7): whoever understands that the soul's nature is that of a substance and not of a body will see that people who think it bodily are misled by the images of bodies they cannot think without.
Respondeo
The argument is short and turns on knowledge.
By the intellect a human being can know the natures of all bodies. But whatever knows certain things cannot have one of them in its own nature, because what is in it naturally would get in the way of knowing the others. Aquinas's example: a fevered tongue, coated with something bitter, tastes everything as bitter. If the intellect were a body, it would have the determinate nature of that body and could not know all bodies.
For the same reason it cannot understand through a bodily organ, as the eye sees through the pupil. A tinted glass makes everything poured into it look the same colour. An organ would tint every act of understanding.
So the intellect has an operation of its own, apart from the body. And only what subsists can operate of itself: we do not say heat heats, but the hot thing heats. Therefore the human soul, which is called intellect or mind, is incorporeal and subsistent.
The replies
- To the first. "This particular thing" has two senses. In one it means anything that subsists and is not a mere quality of something else; in that sense the soul is a "this". In the other it means something complete in its species; in that sense only the whole man, soul and body, is a "this". The soul is a part of human nature, as a hand is a part, but unlike a hand it subsists.
- To the second. Aristotle was reporting others' view. And even granting the point, the better way to say it is that the soul understands as the eye sees: strictly, "man understands through the soul".
- To the third. The body is needed for understanding on the side of the object, not as the organ that understands. The image from the senses is to the intellect what colour is to sight. An animal needs things outside it to perceive, and it is still a subsisting thing.
What the article does not claim
Read on, because a common misreading starts here.
- The soul is not the whole person. In q. 75, a. 4 Aquinas argues that a man is not his soul alone but a composite of soul and body. Sensing is an act of the whole composite, so a human being is not a mind using a body.
- The soul is the form of the body. In q. 76, a. 1 he holds, with Aristotle (De Anima ii, 2), that the intellectual soul is the form of the human body. One soul makes this matter a living, sensing, understanding human being. Aquinas is not a dualist of two things stuck together.
- Immortality follows, but the argument is about operation. In q. 75, a. 6 he argues that the human soul is incorruptible: what has being of itself cannot lose it by the corruption of something else. He adds a sign from desire: every intellectual being naturally desires to exist always, and a natural desire is not in vain.
How it lines up with the Catechism
The Catechism teaches that the human person is "a being at once corporeal and spiritual" (CCC 362), that "soul" signifies "the spiritual principle in man" (363), and that the soul is the "form" of the body, so that "spirit and matter, in man, are not two natures united, but rather their union forms a single nature" (365). It also teaches that each spiritual soul is created immediately by God and is immortal, and will be reunited with the body at the resurrection (366).
Common questions people bring to this article
"Does Aquinas think the soul is in the brain?" No. The soul is the form of the whole body, not housed in one organ. He does hold that understanding depends on images from the senses, so damage to the body can impede thinking without the intellect itself being a bodily organ.
"Is this Plato?" No. Plato tended to treat the person as a soul using a body. Aquinas argues against that in q. 75, a. 4 and q. 76, a. 1.
"Do animals have souls?" Yes, in Aquinas's sense: every living thing has a soul as its principle of life. Only the human soul has an operation, understanding, that does not use a bodily organ, and so only it subsists (q. 75, a. 3).
How to work this article in Aquinian Studio
Put the question in disputation form: "Whether the human soul can understand without the body?" Aquinian distinguishes the terms first (subsist, operate, organ, phantasm), orders the authorities, and states the strongest objection before it determines anything. Investigate then treats that answer as the object of a second pass. It reasons with authority; it does not become authority, and it is not a priest. Check named passages against the primary text before formal use. aquinian.com. New accounts start with 150 free credits, no card.
FAQ
What does Aquinas say the soul is? The first principle of life in a living body and the form of the body. The human soul is also incorporeal and subsistent, because understanding is an act that does not use a bodily organ (Summa Theologiae I, q. 75, aa. 1-2; q. 76, a. 1).
Does Aquinas believe the soul is immortal? Yes. In Summa I, q. 75, a. 6 he argues the human soul is incorruptible, because it has being of itself and cannot lose it when the body corrupts.
Is the soul the same as the person for Aquinas? No. A human being is soul and body together (Summa I, q. 75, a. 4). The soul is part of human nature, though a part that subsists.
Sources
Summa Theologiae I, qq. 75-76, Fathers of the English Dominican Province translation (1920), newadvent.org/summa/1075.htm and 1076.htm. Catechism of the Catholic Church 362-368, vatican.va/archive/ENG0015/__P1B.HTM.