If you bring up the topic of the soul, it’s not uncommon for folks to give you a blank stare. And even if folks do have something to say about it, they often think of it as some separate thing in us that’s interacting with our body—like how a puppeteer might manipulate a puppet or a poltergeist might maneuver a body as its own.
But this is far from what the soul is.
To get a proper understanding of what we’re talking about, let’s start with two simple things: a rock and a plant. Is there a difference between the two? Any kid will tell you there is. The plant is alive; the rock is not.
So there’s something to the plant that makes it a living thing rather than a non-living thing. St. Thomas Aquinas, and Aristotle before him, identifies that something as the soul, “the first principle of life of those things which live” (Summa Theologiae I:75:1).
Therefore, everything that is living—plants, animals, humans, and all the rest (e.g., fungi, monerans)—has a soul and lives because of a soul. The soul is what makes a thing a living being.
But not all souls are created equal. In fact, plant souls, animal souls, and human souls all belong to different orders. These are called the vegetative, sensitive, and rational orders.
In 1914 and 1916, the Church’s ordinary Magisterium confirmed this truth when it published in the Acta Apostolica Sedis (the official journal of the Holy See) a list of twenty-four theses derived from the theological and philosophical tradition of Aquinas. Thesis 14 reads as follows:
Souls in the vegetative and sensitive orders cannot subsist of themselves, nor are they produced of themselves. Rather, they are no more than principles whereby the living thing exists and lives; and since they are wholly dependent upon matter, they are incidentally corrupted through the corruption of the composite.
Regardless of what order of soul we’re talking about, the next thing we need to know about a soul is that it is the form of a body. Aquinas follows Aristotle on this (ST I:76:1). The Catechism even adopts this explanation, enshrining it in official Catholic teaching:
The unity of soul and body is so profound that one has to consider the soul to be the “form” of the body: i.e., it is because of its spiritual soul that the body made of matter becomes a living, human body (365).
Form is just a word we use to signify that which makes a thing the kind of thing it is. For example, when we look at a table, it may happen to be made of wood or iron, but regardless of the material used to construct it, it nonetheless has the form of a table. In other words, it’s not a chair, a plate, a fork, a spoon, etc.—it’s a table. Form is the organizational pattern that makes the matter of a thing what it is—in this case, a table.
A soul is a form that makes a living thing the kind of living thing it is—a plant, an animal, a human person. It’s “the organizational pattern or form of all the parts and all the parts of all the parts,” coordinating the matter to be the kind of living thing it is. The soul of a plant informs and makes the plant’s matter that of a plant. The soul of a lion informs and makes the lion’s matter that of a lion. The soul of a human being informs and makes the human’s matter that of a human being.
By contrast, the matter of a plant that has died is no longer that of a plant. Immediately after death, the matter takes on new distinct forms. What these new forms are exactly may be hard to discern. But we do know they are now actually individual material substances that accidentally constitute what we see to be a single thing. These individual material substances would have been present virtually (not present as an actual substance) in the plant only before the loss of its soul. We may still call it a plant—but given that it no longer has its life principle to unify the matter and allow it to operate as plants do, the matter is no longer that of a plant. Likewise with a dead lion or human being. It’s the soul, then, that makes the body not only a living body, but the kind of living body it is.
Now, there are a couple of important points about the soul that follow from it being the form of the body. One is that the soul is not a separate substance from the body, like a ghost trapped in the machine of the body. Rather, the soul and body together (whether for a plant, an animal, or a human being) make up one thing—one substance.
We see that this is true by considering how the soul is the first principle of life not only in a thing, but also in all a thing’s activities. As the form of a living thing, the soul makes a thing what it is. Being a particular kind of thing involves having certain powers and activities that go with being the kind of thing it is. So a plant does what a plant does—takes in nutrients and grows. An animal does what’s proper to animals—like plants, it takes in nutrients and grows, but unlike plants, it senses and has the power to move. Human beings do what’s proper being a human—take in nutrients, grow, sense, move, and rationally know and love.
Since the soul makes a thing what it is, and since being a particular kind of thing involves having certain powers and activities, it follows that the soul is the seat of all of a living thing’s powers and activities.
Now, as Aquinas argues, vegetative and sensory powers and activities (which plants, animals, and humans have) belong to the bodies of corporeal beings (ST I:75:3). Since the soul is the seat of those bodily powers and activities, it follows that vegetative and sensory powers and activities proceed by way of both body and soul. And since these activities are of one thing—an action being performed by a single thing (the plant growing, the lion running, the human seeing)—it follows that body and soul together form one thing.
Another point is that the soul is entire in the whole body and in each of its united parts. A branch that’s cut from the tree, for example, no longer has the form of the tree. The matter takes on new distinct forms and thus becomes a conglomeration of individual material substances, just as the matter of the whole tree would if it were to die. The same goes for a limb that’s cut off from a human body: the cut off hand is no longer a human hand because it no longer has the person’s soul as its form. So there’s no dividing up the soul.
Given the different powers and activities that each order of souls allows for, we can see a certain hierarchy. As we move from plants to humans, we see the powers climb the ladder of perfection: nutrition and growth to sensation and self-local motion to rational knowledge and love.
There are many more questions that arise concerning the nature of souls. Can they exist without the body? Even if some can exist without the body, can they be destroyed? These we’ll have to save for some other time. But suffice it to say for now that as the form of a body, the soul is not all that mysterious after all.