Commentary Aristotle Physics
Book 2. The Principles of Natural Science
Lecture 1
What is nature, and what things have it?
Of things that exist, some exist by nature and some from other causes. “By nature,” the animals and their parts exist, and the plants and the simple bodies (earth, fire, air, water)—for we say that these and the like exist “by nature.”
All the things mentioned present a feature in which they differ from things that are not constituted by nature. Each of them has within itself a principle of motion and of stationariness (in respect of place, or of growth and decrease, or by way of alteration).
On the other hand, a bed and a coat and anything else of that sort, by merit of receiving these designations—that is, insofar as they are products of art—have no innate impulse to change. But, insofar as they happen to be composed of stone or of earth or of a mixture of the two, they do have such an impulse, and just to that extent.
And this seems to indicate that nature is a principle and cause of motion and rest in that in which it is primarily and per se, and not per accidens.
I say, “not per accidens,” because (for instance) a man who is a doctor might cure himself. Nevertheless, it is not insofar as he is a patient that he possesses the art of medicine: it merely has happened that the same man is doctor and patient—and that is why these attributes are not always found together.
So it is with all other artificial products. None of them has in itself the source of its own production.
But, while that principle is in something else external to the thing in some cases (for instance, houses and the other products of manual labor), in other cases—those that may cause a change in themselves accidentally—it lies in the things themselves (but not in virtue of what they are). Nature, then, is what has been stated.
Things have a nature that have a principle of this kind. Each of them is a substance, for it is a subject, and nature always implies a subject in which it inheres.
The term “according to nature” is applied to all these things and also to the attributes that belong to them in virtue of what they are—for instance, the property of fire to be carried upward, which neither is a nature nor has a nature, but is “by nature” or “according to nature.” What nature is, then, and the meaning of the terms “by nature” and “according to nature,” has been stated.
That nature exists it would be absurd to try to prove; for it is obvious that there are many things of this kind, and to prove what is obvious by what is not is the mark of a man who is unable to distinguish what is self-evident from what is not. (This state of mind is clearly possible. A man blind from birth might reason about colors. Presumably, therefore, such persons must be talking about words without any thought to correspond.)
141. After the Philosopher has treated the principles of natural things in book 1, he here treats the principles of natural science. Now, the things that we ought to know first in any science are its subject and the middle term by which it demonstrates.
Hence book 2 is divided into two parts.
First, he determines what things belong to the consideration of natural science;
second, at “now that we have established” (194b16; [176]), he points out the causes from which it demonstrates.
The first part is divided into two parts.
First, he shows what nature is.
Second, at “we have distinguished” (193b22; [157]), he determines what things natural science considers.
The first part is divided into two parts.
First, he shows what nature is;
second, the number of ways in which nature is used is pointed out, at “some identify” (193a9; [149]).
The first part is divided into two parts.
First, he shows what nature is.
Second, at “that nature exists” (193b3; [148]), he refutes the position of those who attempt to demonstrate that nature exists.
Concerning the first part, he makes two points.
First, he states what nature is;
second, at “things have a nature” (192b32; [146]), he designates those things which are called “nature.”
Concerning the first part, he makes three points.
First, he inquires into the definition of nature;
second, he arrives at “the definition at and this seems to indicate” (192b20; [145]);
third, he explains this definition at “I say” (192b23; [145]).
142. He first says, therefore, that we say that, of all beings, some are from nature, while others are from other causes, such as from art or from chance. Now, we say that the following things are from nature: every sort of animal, and their parts, such as flesh and blood, and also plants and simple bodies (namely, the elements, such as earth, fire, air, and water, which are not resolved into any prior bodies). For these and all things like them are said to be from nature. All of these things differ from the things that are not from nature, because all things of this sort seem to have in themselves a principle of motion and rest: some according to place (such as the heavy and the light, and also the celestial bodies), some according to increase and decrease (such as the animals and plants), and some according to alteration (such as the simple bodies and everything that is composed of them). But things that are not from nature, such as a bed and clothing and like things, which are spoken of as not from nature because they are from art, have in themselves no principle of change except per accidens, insofar as the matter and substance of artificial bodies are natural things. Thus, insofar as artificial things happen to be iron or stone, they have a principle of motion in them, but not insofar as they are artifacts. For a knife has in itself a principle of downward motion not insofar as it is a knife, but insofar as it is iron.
143. But it does not seem to be true that, in every change of natural things, a principle of motion is in that which is moved. For in the alteration and the generation of simple bodies, the whole principle of motion seems to be from an external agent. For example, when water is heated or air is converted into fire, the principle of the change is from an external agent.
Therefore, some say that, even in changes of this sort, an active principle of motion is in that which is moved, not perfectly, but imperfectly, which principle helps the action of the external agent. For they say that, in matter, there is a certain inchoateness of form, which they say is privation, the third principle of nature. And the generations and alterations of simple bodies are said to be from this intrinsic principle.
But this cannot be. Since a thing acts only insofar as it is in act, the named inchoate state of form cannot be an active principle, since it is not act, but a certain disposition for act. And, furthermore, even if it were a complete form, it would not act on its own subject by altering it. For the form does not act; rather, the composite acts. And the composite cannot alter itself unless there are two parts in it, one that alters and the other that is altered.
144. And so it must be said that a principle of motion is in natural things in the way in which motion belongs to them. Therefore, in those things to which it belongs to move, there is an active principle of motion. But in those things to which it belongs to be moved, there is a passive principle, which is matter. And this principle, insofar as it has a natural potency for such a form and motion, makes the motion to be natural.
And, for this reason, the production of artificial things is not natural. For even though the material principle is in that which comes to be, it does not have a natural potency for such a form. So also the local motion of the celestial bodies is natural, even though it is from a separated mover, inasmuch as there is in the celestial body itself a natural potency for such a motion. However, in heavy and light bodies, there is a formal principle of motion. (But a formal principle of this sort cannot be called the active potency to which this motion pertains. Rather, it is understood as a passive potency. For heaviness in earth is not a principle for moving, but rather for being moved.) For just as the other accidents are consequent upon substantial form, so also is place, and thus also to be moved to place. However, the natural form is not the mover. Rather, the mover is that which generates and gives such and such a form upon which such a motion follows.
145. Next, at “and this seems to indicate” (192b20), he concludes from the above the definition of nature in the following manner. Natural things differ from the non-natural insofar as they have a nature. But they differ from the non-natural only insofar as they have in themselves a principle of motion. Therefore, nature is nothing other than a principle and cause of motion and rest in that in which it is primarily and per se, and not per accidens.
Now principle is placed in the definition of nature as a sort of genus, and not as something absolute, for the name “nature” involves a relation to a principle. For those things are said to be born that are generated after having been joined to a generator, as is clear in plants and animals. Thus, the principle of generation or motion is called “nature.” Hence they are to be laughed at who, wishing to correct the definition of Aristotle, tried to define nature by something absolute, saying that nature is a power seated in things or something of this sort. Moreover, nature is called a principle and cause in order to point out that, in that which is moved, nature is not a principle of all motions in the same way, but in different ways, as was said above.
Moreover, he says that nature is a principle of motion and rest. For those things that are naturally moved to a place similarly, or even more so, naturally rest in that place. Because of this, fire is naturally moved upward, since it is natural for it to be there. And, for the same reason, everything can be said to be moved naturally and to rest naturally in its place. Nevertheless, this must not be understood to mean that, in everything that is moved naturally, nature is also a principle of coming to rest. For a heavenly body is indeed moved naturally, but it does not naturally come to rest. But, on the whole, it can be said that nature is a principle not only of motion but also of rest.
Further, he says in which it is in order to differentiate nature from artificial things, in which there is motion only per accidens. Then he adds primarily because, even though nature is a principle of the motion of composite things, it nevertheless is not such primarily. Hence, an animal is moved downward not because of the nature of animal insofar as it is animal, but because of the nature of the dominant element.
He explains why he says, per se and not per accidens, at I say “not in virtue of” (192b23). It sometimes happens that a doctor is the cause of his own health, and so the principle of his own coming to health is in him, but per accidens. Hence, in him the principle of coming to health is not nature. For it is not insofar as he is cured that he has the art of medicine, but insofar as he is a doctor. Hence, the same being happens to be a doctor and to be cured, and he is cured insofar as he is sick. And, so, because these things are joined per accidens, they are also at times separated per accidens. For it is one thing to be a doctor who cures, and another thing to be a sick person who is cured.
But the principle of a natural motion is in the natural body that is moved insofar as it is moved. For insofar as fire has lightness, it is carried upward. And these two things are not divided from each other such that the lightness is different than the body that is moved upward. Rather, they are always one and the same.
And all artificial things are like the doctor who cures. For none of them has in itself the principle of its own making. Rather, some of them come to be from something outside, as a house and other things that are carved by hand, while others come to be through an intrinsic principle, but per accidens, as was said. And so it has been stated what nature is.
146. Next, at “things have a nature” (192b32), he defines those things that are given the name “nature.” He says that those things that have in themselves a principle of their motion have a nature. And such are all subjects of nature. For nature is a subject, and so it is called matter, and nature is in a subject, and so it is called form.
147. Next, at the term “according to nature” (192b35), he explains what is according to nature. He says that to be according to nature is said both of subjects whose existence is from nature and also of the accidents that are in them and caused by such a principle. Thus, to be carried upward is not a nature itself, nor does it have nature, but it is caused by nature. And thus it has been stated what nature is, and what it is that has nature, and what is according to nature.
148. Next, at “that nature exists” (193b3), he denies the presumptuous position of those who wish to demonstrate that nature exists. He says that it is ridiculous for anyone to attempt to demonstrate that nature exists. For it is manifest to the senses that many things are from nature that have in themselves the principle of their own motion. Moreover, to wish to demonstrate the obvious by what is not obvious is the mark of a man who cannot judge what is known in itself from what is not known in itself. For when he wishes to demonstrate that which is known in itself, he uses that which is known in itself as if it were not known in itself.
And it is clear that some people do this. A man who is born blind may sometimes reason about colors. But that which he uses as a principle is not known to him per se, because he has no understanding of the thing. Rather, he only uses names. For our knowledge has its origin from the senses, and he who lacks one sense lacks one science. Hence those who are born blind and who never sense color cannot understand anything about color, and so they use the unknown as if it were known. And the converse applies to those who wish to demonstrate that nature exists. For they use the known as if it were not known. The existence of nature is known per se, insofar as natural things are manifest to the senses. But what the nature of each thing is, or what the principle of its motion is, is not manifest.
Hence it is clear from this that Avicenna unreasonably attempted to disprove what Aristotle has said, wishing that it were possible to prove the existence of nature. However, Avicenna did not wish to prove this from natural things, for no science proves its own principles. But ignorance of moving principles does not mean that the existence of nature is not known through itself, as was said.