8.2 Aquinas on Life
What is the difference between something that is alive and something that is not alive? Aquinas’s answer to this question is that living things, unlike nonliving things, are self-moving and have within themselves an active principle of movement, or an ability of their own to move themselves or others.
You might think that nonliving things can move since they can cause other things to undergo motion in any of the senses of motus that I noted Aquinas to subscribe to when I was explaining his understanding of motus in chapter 3. Yet Aquinas does not suppose otherwise, as we can see from the ways in which he speaks of agent causation in general. Of course, he thinks that a tennis ball can be moved by a nonliving tennis racket, and so on. But he does not think of a tennis racket as being self-moving.
Yet have we not now seen that Aquinas does not think of anything other than God as being a wholly unmoved mover? Indeed, we have. So what sense of “self-moving” does Aquinas have in mind when thinking of things in the world as living as opposed to nonliving? It is the sense in which something can be said to undergo change, or to initiate change in something else, without being pushed around by something else in the world. It is the sense in which, and forgetting about God, something might literally be said to be an automobile, even if it is not a car.
Aquinas accepts that things other than human beings are alive. He also thinks of nonhuman animals and plants as having life. In his view, all of these can operate or go through various changes by virtue of what they naturally are, as he thinks that nonliving things cannot. You might say that things that have died can go through changes because of what they are, that corpses, for instance, can decay. Yet, so Aquinas would reply, corpses are not what living things that become corpses are. On his account, when I die, then I cease to exist as a human being. My corpse, he thinks, is not the living thing that I am, and the corpse of a cat or what remains of a once living plant is neither a cat nor a plant. Corpses and the remains of plants obviously undergo change. But this, for Aquinas, is not a change in a human being or a cat or a cabbage, or whatever. It is change in what used to be a human being or a cat or a cabbage but is not now.
In saying this kind of thing Aquinas is drawing on his notion of substantial form. For him, “X is a cat” is true if X has what it takes to be a cat as opposed to something else. Or “X is a human being” is true if X has what it takes to be a human being. Since Aquinas does not regard a dead cat as a cat or a dead human being as a human being, he does not think of feline corpses as cats or of dead human beings as human beings. He thinks that something is a cat or a human being or a cabbage because it has a particular substantial form, and his distinction between living and nonliving things relies on this view. Living things, he says, are things with a certain kind of substantial form, things with a certain way of being or existing.
What kind of substantial form? As I presume you will appreciate, Aquinas thinks that an ideal observer’s list of what exists with particular substantial forms is going to be incredibly long. That is because he thinks that many different things exist with different substantial forms. Yet he also thinks that the list can be divided into things that are alive and things that are not alive since some things move of themselves while others do not. He thinks, for example, that cats are alive because they can prowl around without being dragged or tinkered with by something else in the world. Correspondingly, he thinks that the corpse of a cat is not alive since any movements or changes it undergoes are not initiated by it in any sense but are explicable in terms of what is acting on it from outside. Aquinas certainly holds that a living cat is not a totally unmoved mover and that no living thing in the world is wholly unmoved. That is because he thinks that behind all movement or change is the mover he refers to as God, whom he takes to be the primary unmoved mover. Nevertheless, Aquinas holds that we can distinguish between cats, people, and cabbages on the one hand and desks, rugs, and stones on the other. The former, he holds, move of themselves in various ways, while the latter do not. He maintains that cats, people, and cabbages can undergo changes explicable in terms of what they are, not changes only explicable in terms of things other than them acting on them and thereby indicating what they are. So, for example, in Summa Theologiae 1a,18,1 he writes, echoing Aristotle:
We first say that an animal is living when it begins to have movement of itself, and we judge it to be alive so long as this kind of movement appears in it. When it no longer has any movement of itself, but is moved only by another, we say that life has failed and that the animal is dead. So, it is clear that only things which move themselves with one or another kind of movement are, properly speaking, alive. … Thus, we call “living” those things which produce in themselves some kind of movement or operation. It is only metaphorically that we can say that things whose nature does not contain that power are living things.4
I should add that, corresponding to our distinction between “animate” and “inanimate,” Aquinas, like Aristotle, holds that anything living has a soul (anima), meaning, and only meaning, that souls come to exist in the world as any living things come to exist in it.