Aquinas and Plato on the Soul, Body, and Human Nature

The Soul and the Human Species in Aquinas

Thomas Aquinas explains his statement, "the soul is man," in two senses. In the first sense, a man is composed not just of the body but also of the soul. Some suggest that "form alone belongs to the species, while matter is part of the individual, and not the species." Aquinas denies this. He maintains that the nature of a particular species is what its definition signifies, and that natural things are defined according to both their form and matter. Matter, according to him, is part of a particular species and is shared by all others under that same species: a particular man is composed of a soul, flesh, and bones, just as other men or individuals in common are. What belongs to the substance of a particular man also belongs to the substance of other individuals belonging to the same given species (Aquinas 1947).

The second sense in which the statement can be understood concerns the sensitive operations of the soul. If the operations of the sensitive soul of a given man were proper to it alone — as all such operations are attributable to man — then whatever performs those operations proper to that soul is that thing. In other words, if what the sensitive soul does is proper to it, apart from the body, because all such actions that man performs are performed by the soul only, then that which performs acts proper only to a man is a man, and not a tree or a stone.

However, the capacity to sense is an operation not only of the soul but also of the body. Sensation, Aquinas says, is an operation performed by man, but is not proper to him alone. Animals and plants can sense as well.

Aquinas further clarifies the distinction between the "inward" man and the "outward" man. In man there is an intellectual part, which is in accordance with truth; this he refers to as the "inward" man. The sensitive part of man he calls the "outward" man — the man that those whose observations of life do not exceed the physical senses take to be the whole of man.

He also refers to the hypostasis of the soul and body in the human species. This hypostasis does not refer to every particular substance, such as a hand or a foot, but to that which possesses the complete nature of its species.

The Soul's Distinction from Divine Nature

Aquinas states that the human soul cannot share or possess the same substance as the Divine. The Divine Nature is a pure act that receives or needs nothing from any other being and "admits of no variety in itself." The human soul, by contrast, remains in a state of potentiality in the exercise of intelligence. It can acquire knowledge only from things and possesses various powers incompatible with those of the Divine Nature.

Aquinas traces this error — the belief that the human soul shares the Divine substance — to two ancient groups. The first could not rise above the imagination and assumed that only bodies existed. This group believed that God was a body and that God's body was the principle of all other bodies. These thinkers held that the soul had the same nature as the body, which they considered the first principle. This claim is found in De Anima (Aristotle 1993), leading them to conclude that the human soul had the same nature as God's.

The second group consisted of the Manichaeans, who believed that God was corporeal or bodily light. They held that the human soul was part of that light and bound up with the human body. This belief advanced into suggesting the existence of something non-bodily — not apart from the human body, but rather the form of a body. Hence, their representative claimed that "God is a soul governing the world by movement and reason."

This conclusion led them to suppose that the human soul was part of the one governing soul and that man is therefore a part of the whole world. These proponents could not comprehend the different degrees of spiritual substance as distinct from the distinctions among bodies. Aquinas declares all these theories false, because the human soul is not of the same substance as God.

The Soul's Origin and Its Union with the Body

Aquinas retains Origen's supposition that the souls of all men were created from the first at the same time as the angels and that both were equal in their natural condition, differing only according to merit. Some of them had bodies, like those of men, while others remained free of matter. He also shares the opinion of St. Augustine that the human body was produced not actually but only by virtue of certain "causal virtues" — not inherent to the soul — because neither body nor soul was made of any pre-existing bodily or spiritual matter, nor could either be produced from any created virtue.

Aquinas therefore suggests that, in the first six days of God's creation of the universe, the human soul was created together with the angels and was later, by God's own will, joined to the service of the body. The sequence of events in those first six days, Aquinas says, indicates that the human soul was already in existence when the human body was created.

This succession, in Aquinas's view, underscores the complete separation of the species and natures of the body and the soul of man, and that the soul is united to the body not as its form but as its ruler or administrator. As such, the soul became a natural part of human nature. He recognized that the human soul was a complete species in itself at the beginning, but since it is the natural form of the human body, reason points to its having been created not separately but within the human body.

Bibliography

Aquinas, Thomas. Summa Theologica. Translated by the Fathers of the English Dominican Province. Benzinger Brothers edition, 1947. http://www.cccel.org/a/aquinas/summa.home.html

Aristotle. De Anima (Books II and III). Translated by D. W. Hamlyn. Paperback. Clarendon Press reprint, 1993.

Plato. Phaedo (360 BC). Translated by Benjamin Jowett. http://www.constitution.org/pla/phaedo.htm