LECTURE 1 (192 b 8-193 a 8)

WHAT IS NATURE? WHAT THINGS HAVE A NATURE? WHAT THINGS ARE 'ACCORDING TO NATURE'?

             141. After the Philosopher has treated the principles of natural things in Book I, he here treats the principles of natural science.

             Now the things which we ought to know first in any science are its subject and the method by which it demonstrates.

             Hence Book II is divided into two parts. First he determines what things belong to the consideration of natural science, and secondly, where he says, 'Now that we have established . . .' (194 b 16), he points out the causes from which it demonstrates.

             The first part is divided into two parts. First he shows what nature is. Secondly, where he says, 'We have distinguished . . .' (193 b 23), he determines what things natural science considers.

             The first part is divided into two parts. First he shows what nature is. Secondly the number of ways [in which the name nature is used] is pointed out, where he says, 'Some identify . . .' (193 a 9).

             The first part is divided into two parts. First he shows what nature is. Secondly, where he says, 'That nature exists . . .' (193 a 2), 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. Secondly, where he says, 'Things "have a nature" . . .' (192 b 33), he designates those things which are called 'nature'.

             Concerning the first part he makes three points. First he inquires into the definition of nature. Secondly he arrives at the definition, where he says, '. . . nature is . . .' (192 b 22). Thirdly, he explains this definition, where he says, 'I say . . .' (192 b 23).

             142. He says, therefore, first that we say that of all beings some are from nature, whereas others are from other causes, for example, 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, i.e., 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 which 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 which is composed of them.

             But things which are not from nature, such as a bed and clothing and like things, which are spoken of in this way because they are from art, have in themselves no principle of mutation, 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 aforesaid inchoate state of form, since it is not act, but a certain disposition for act, cannot be an active principle. And furthermore, even if it were a complete form, it would not act on its own subject by changing 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 of which alters, the other of which 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. Whereas 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 where he says, '. . . nature is . . .' (192 b 22), 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 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 its genus, and not as something absolute, for the name 'nature' involves a relation to a principle. For those things are said to be born which 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 which are naturally moved to a place, also or even more 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. This, however, must not be understood to mean that in everything which 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 not only a principle 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, nevertheless it is not such primarily. Hence that an animal is moved downwards is 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' where he says, 'I say "not in virtue of . . ." ' (192 b 24).

             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 nature is not the principle of his coming to health. 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 which 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 so that the lightness is different than the body which 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 which 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 where he says, 'Things "have a nature" . . .' (192 b 33), he defines those things which are given the name 'nature'.

             He says that those things which have in themselves a principle of their motion have a nature. And such are all subjects of nature. For nature is a subject insofar as it is called matter, and nature is in a subject insofar as it is called form.

             147. Next where he says, 'The term "according to nature" . . .' (192 b 35), 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 which 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 where he says, 'That nature exists . . .' (193 a 2), 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, which have in themselves the principle of their own motion. To wish, moreover, to demonstrate the obvious by what is not obvious is the mark of a man who cannot judge what is known in itself and 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 colours. 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 colour, cannot understand any thing about colour. 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, who wished that it were possible to prove the existence of nature, unreasonably attempted to disprove what Aristotle has said. 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 per se, as was said.