Lecture 3 — The definition justified and the modes of living

Since it is from things uncertain (though more certain [to us]) that what is certain and more known according to account arises, in this way we must try to approach the matter afresh.

For not only should a defining account say the fact, as do most determinations, but it should also contain and demonstrate the cause. At present the accounts of the determinations are like conclusions: for instance, what is the square equal to a rectangle? The equilateral rectangle. Such a determination has the account of a conclusion. But to say that the square is the discovery of the mean line states the thing’s cause.

Going back, then, to the beginning of our inquiry, let us say that the ensouled is distinguished from the non-souled by being alive.

To live, however, is predicated in several ways; and we say there is life even if only one of these is present, as, for example, understanding, sensation, or movement and rest in place, as well as the movement and rest involved in nourishment and growth and decrease.

Hence all plants are also seen to live. For they appear to have in themselves a power and principle of this kind, by which they admit of increase or decrease in various directions—For they do not only grow up or down, but likewise grow in both directions. And in all their parts they are continually nourished, and they live so long as they can take in nourishment.

It is possible for this power to be separate from the others, whereas for the others to be separate from it is impossible, at least in mortal beings. This is evident in plants, for there is in them no other power of the soul. To live by this principle, then, is common to all living things.

But an animal is such primarily by sensation. For we also call “animals” things that do not move or change their place, provided they have sensation and do not merely live.

There seem to be many animals of this sort: by nature they stay in one place, but they have only sensation.

But in all of them, the first of the senses is touch. Just as the vegetative can be separated from touch and all the senses, so can touch from the other senses. We give the name “vegetative,” however, to that part of the soul in which vegetables also participate. But all animals are seen to possess the sense of touch.

Due to what cause each of these facts is so we shall say later on. At present, only this need be said: that soul is the principle of things we have discussed and is determined by them, that is, by the vegetative, sensitive, intellectual, and motive powers.

245. Since it is from things uncertain (413a11). Having presented the definition of the soul, the Philosopher now sets out to demonstrate it.

First, he says what he intends to do;

and second, at going back, then (413a20; [253]), he proceeds to do it.

About the first, he does two things:

first, he determines the method of demonstration that he intends to use;

second, at for not only should (413a13; [247]), he explains how certain types of definition can be demonstrated.

With regard to the first, we should note that, since we can only come to know the unknown if we start from what we know, and since the purpose of demonstration is precisely to cause knowledge, it follows that every demonstration must begin from something more knowable to us than the thing the demonstration makes known.

Now, in some subjects the things more known to us are also such according to their nature, as with mathematical objects, which have been abstracted from matter; and in these things, demonstration can proceed from things simply speaking more known and more known according to their nature, that is, it can proceed from causes to effects; whence also this demonstration is called “that on account of which.”

But in some other things what is more known simply speaking is not the same thing as the more known to us, as happens with natural objects; for sensible effects are usually more known than their causes. Hence, in the natural sciences we for the most part have to begin from things less known according to their nature but more known to us, as is said in Physics 1. (Aristotle, Physics 1.1, 184a16–21; cf. Aquinas, Commentary on Physics, bk. 1, lect. 1.)

246. And this is the kind of demonstration which will be used here. So he says that what is according to its nature certain and more known according to its account (413a11) becomes more certain to us by means of things uncertain according to their nature but more certain to us; and that this shows us the method whereby one must try to approach the soul afresh by demonstrating the definition given above. (Lect. 1, n. 233.)

247. Then, at for not only should (413a13), he tells us why the question must be taken up again by showing that certain definitions can be demonstrated. And this is why he says that we should approach the soul afresh, because it is not enough for the defining account to say, as most determinations (that is, definitions) do, the fact; it should also touch on the cause of the fact; with the definition saying the “that on account of which,” one can then demonstrate the definition that states the mere fact.

At present there are found many accounts of determinations (that is, definitions) that are like conclusions; and he gives an example from geometry.

248. To understand which, we must note that there are two kinds of four-sided figure: those whose angles are all right angles, and these are called “rectangles,” that is, an area of right angles; and those with no right angles, and these are called “rhombuses” or “rhomboids.”

Note, however, that of the rectangles there is one with four equal sides—called the “square” or “tetragon”; and another which, without having all four sides equal, has two pairs of equal and opposite sides—and this rectangle is called the “oblong,” where it is longer in one direction than in the other. Thus:

249. Note further, that in any rectangular area, the two straight lines enclosing the right angle are said to contain the whole area, because, the other two sides being equal to these two, each equal to its opposite, it follows that one of the enclosing lines measures the length of the whole area, and the other its breadth, so that the whole rectangular area arises from leading one line against the other. For example, if we imagine one of these lines moving along the other, we see the whole area come forth.

250. Note also that since, in an oblong rectangle, the two sides that contain it are unequal, if one takes the proportional mean between them, and leads it against itself, one gets a square equal to the oblong rectangle. And because showing this by the geometrical demonstrations would be diffuse, let the following manifestation in numbers be enough for the present.

Let our oblong rectangle, then, have its longer side nine spans and its shorter side four spans. Then let us take the proportional mean between them, six spans. (That is, as nine is to six, so is six to four.) Now the square of this line must equal the aforesaid oblong rectangle, which is also clear in the numbers: for four times nine are thirty-six, and likewise, six times six are thirty-six.

251. This is why he says that the question, what is the square equal to an oblong rectangle? is given this definition: a rectangle, that is, a surface composed of right angles, that is equilateral, that is, has all its sides equal. Such a determination, that is, such a definition, has the account of a conclusion, that is, of a presupposed demonstration. But if one were to define it saying that the square is the discovery of a mean line, that is, of the proportional mean between the two unequal sides of the oblong, that is, the square is the rectangle constructed from this discovered line, then this definition says, that is, defines, the thing’s cause.

252. Note, however, that this example is only relevant to the definition of the soul in a certain respect, that is, insofar as the definition of the soul is to be demonstrated; it must not be taken to imply that it can be demonstrated by a demonstration that states the “that on account of which.”

253. Next, at going back then (413a20), he begins to demonstrate the definition of the soul given above, and this in the way indicated, that is, through its effect. (Lect. 1, n. 233.) He uses the following demonstration: the first principle of living is the act and form of living bodies; but soul is the first principle of living in living things; therefore, the soul is the act and form of a living body. Now, this demonstration is clearly from what is posterior; for the soul is the principle of the activities of life because the soul is the form of a living body, and not the reverse. He therefore has to do two things here:

first, to show that soul is the principle of living;

and second, to show that the first principle of living is the form of the living body; this comes at since we can (414a4; [271]).

With regard to the first point, he does three things:

first, he distinguishes modes of life;

second, he shows that the soul is the principle of living—at hence all plants (413a25; [256]);

third, he explains how the parts of the soul, by means of which it is the principle of the activities of life, are related. This is at we now ask whether each of these (413b13; [262]).

254. He starts, then, by first saying that to carry out our intention of demonstrating the definition of the soul, we must take as a sort of principle that things with souls differ from the non-souled by being alive (413a20). For ensouled things live, but things without souls do not live. But as there are several ways of living, if a thing has life in only one of these ways, it is still said to be alive and to possess a soul.

255. Living, he says, shows itself in four modes: first, through understanding; second, through sensation; third, through local motion and rest; fourth, through the motions of taking nourishment, and decrease and growth.

Now, he distinguishes only these four modes of living even though he has already distinguished five kinds of the soul’s activities; this is because he wants to distinguish the modes of living according to the degrees of living things, which are distinguished according to these four. (Bk. 1, lect. 14, n. 201.) For in some of the things that are alive, like plants, one finds only self-nourishment and growth and decrease; but some have also sensation without local motion—such are the incomplete animals, like shellfish; some again are found also to have local motion, like in the complete animals, like oxen and horses, which move themselves about; and some are also found to have understanding, that is, human beings. However, the appetitive activity, which makes a fifth, does not make a difference in the degrees of living things, for wherever there is sensation, there is also appetite.

256. Next, at hence all plants (413a25), he shows that a soul is involved in all these modes of living. Concerning this he does three things.

First, he shows how the soul is the principle of living in plants;

second, in animals, at but an animal is such primarily (413b2; [259]);

third, he summarizes, at due to what cause (413b9; [261], what has been said and what remains to be said.

As to the first, he does two things.

First, he shows that the principle of living in plants is a soul. And he says that, since it has been said that whatever has one of the four modes of living mentioned above can be said to live, it follows that all plants are alive; for they all possess within themselves some intrinsic power and principle whereby they admit of the motion of growth and decrease.

257. Now, it is clear that this principle is not nature but rather soul, because nature does not move in contrary directions, whereas growth and decrease are in contrary directions. For all plants grow not only upwards or downwards, but in both directions. Hence a soul, not nature, is clearly the principle of their motion. Nor do plants live only when actually growing or decaying, but, in all their parts (413a25) that take nourishment, they live so long as they can take the nourishment that induces growth.

258. Second, at it is possible (413a31), he shows that this principle of living is first and can be separate from the others. And he says that this, namely, the principle of nutrition and growth, can be separate from the other principles of living, but the others cannot be separated from it, at least in mortal beings. He adds this last clause because of immortal beings, like separated substances or heavenly bodies; yet if these are ensouled, the intellectual power is found there without the nutritive.

And the separability of this life principle from others is evident in plants, in which there is no other power of the soul except this. It follows that that due to which life is first found in mortal things is this principle of growth and nourishment, which is called the “vegetative soul.”

259. Then, at but an animal is such primarily (413b2), he manifests how a soul is the principle of living in animals. And here he does two things.

First, he observes that what primarily distinguishes animals is sensation, though there are some animals that both sense and move; for we say those things are animals (not just living things) even if they do not change place, as long as they have sensation. For there are many such animals whose nature restricts them to one place, but which have the power of sense, for example, shellfish, which cannot move from place to place.

260. Then, at but in all of them (413b4), he shows that touch is the primary sense present in animals. He proves this from the fact that just as the vegetative can be separated from touch and all the senses, so touch is separable from the other senses. For there are many animals that have only the sense of touch, like the incomplete animals, whereas all animals have the sense of touch. That part of the soul in which vegetables, that is, plants, also participate we call the “vegetative.”

Hence we can distinguish three degrees of living things: first, that of plants; second, that of the incomplete animals fixed to one place, possessing only the sense of touch; and, third, that of complete animals that move themselves about and which also have the other senses. And a fourth degree consists, evidently, of things that have all this and intellect as well.

261. Finally, at due to what cause (413b9), summarizing what has been said and remains to be said, he remarks that due to what cause each of these facts, namely the separability of the vegetative principle from sensation, and of touch from the other senses, will be given later on. He does this at the end of the whole treatise. (Bk. 3, lect. 11, n. 847 – lect. 12, n. 874.)

At present it suffices to say that the soul is the principle of living according to the aforementioned modes, and which is divided into four, namely, the vegetative mode, which belongs to plants and to all living things; the sensitive mode, which is in all animals; the intellectual mode, in all human beings; and the progressively motive mode, which exists in all the animals completed by the senses or intellect.