Commentary Aristotle Physics
268. Next, at “it is absurd” (199b26), he destroys the third point by which some hold the opinion that nature does not act for the sake of something. For it seems to some that nature does not act for the sake of something because nature does not deliberate.
But the Philosopher says that it is absurd to hold this opinion. For it is obvious that art acts for the sake of something, yet it is also obvious that art does not deliberate. Nor does the artisan deliberate insofar as he has the art, but insofar as he falls short of the certitude of the art.
Hence, the most certain arts do not deliberate, as the writer does not deliberate how he should form letters. Moreover, those artisans who do deliberate do not do so in the execution after they have discovered the certain principles of the art.
Thus, one who plays the harp would seem most inexperienced if he should deliberate in playing any chord. And from this it is clear that an agent does not deliberate, not because he does not act for an end, but because he has the determinate means by which he acts.
Hence, since nature has the determinate means by which it acts, it does not deliberate. For nature seems to differ from art only because nature is an intrinsic principle and art is an extrinsic principle. For if the art of ship building were intrinsic to wood, a ship would have been made by nature in the same way as it is made by art. And this is most obvious in the art that is in that which is moved, although per accidens, such as in the doctor who cures himself. For nature is most like this art.
Hence, it is clear that nature is nothing but a certain kind of art—namely, the divine art—impressed upon things, by which these things are moved to a determinate end. It is as if the ship builder were able to give to timbers that by which they would move themselves to take the form of a ship.
Finally, he concludes by saying that it is clear that nature is a cause and that it acts for the sake of something.
Lecture 15. How necessity is found in natural things
As regards what is “of necessity,” we must ask whether the necessity is hypothetical, or simple as well.
The current view places what is of necessity in the process of production, just as if one were to suppose that the wall of a house necessarily comes to be because what is heavy is naturally carried downward and what is light to the top; thus, the stones and foundations take the lowest place, with earth above because it is lighter, and wood at the top of all as being the lightest.
Nevertheless, though the wall does not come to be without these, it is not because of these except as its material cause: it comes to be for the sake of sheltering and guarding certain things.
It is the same in all other things that involve production for an end: the product cannot come to be without things that have a necessary nature, but it is not due to these except as its material; it comes to be for an end. For instance, why is a saw this way? To effect such-and-such and for the sake of such-and-such.
This end, however, cannot be realized unless the saw is made of iron. It is therefore necessary for it to be of iron if we are to have a saw and perform the operation of sawing.
What is necessary, then, is necessary on a hypothesis; it is not a result necessarily determined by antecedents. Necessity is in the matter, while “that for the sake of which” is in the definition.
Necessity in mathematics is similar in a way to necessity in things that come to be through the operation of nature.
Since a straight line is what it is, it is necessary that the angles of a triangle should equal two right angles, but not conversely—though, if the angles are not equal to two right angles, then the straight line is not what it is either.
But, in things that come to be for an end, the reverse is true. If the end is to exist or does exist, that also that precedes it will exist or does exist; otherwise, just as the premise will not be true if the conclusion is not true, so here the end or “that for the sake of which” will not exist.
For this too is itself a starting-point, but of the reasoning, not of the action; while the starting-point in mathematics is the starting-point of the reasoning only, as there is no action.
If, then, there is to be a house, such-and-such things must be made or be there already or exist, or generally the matter relative to the end (bricks and stones if it is a house). But the end is not due to these except as the matter, nor will it come to exist because of them.
Yet, if they do not exist at all, neither will the house or the saw—the former in the absence of stones, the latter in the absence of iron—just as, in the other case, the premises will not be true if the angles of the triangle are not equal to two right angles.
The necessary in nature, then, is plainly what we call by the name of matter, and the changes in it. Both causes must be stated by the physicist, but especially the end, for that is the cause of the matter, not vice versa.
And the end is “that for the sake of which,” and the beginning starts from the definition or essence.
This is just as in artificial products: since a house is of such-and-such a kind, certain things must necessarily come to be or be there already; or, since health is this, these things must necessarily come to be or be there already.
Similarly if man is this, then these [must necessarily come to be or be there already]; if these, then those.
Perhaps the necessary is present also in the definition.
For if one defines the operation of sawing as being a certain kind of dividing, then this cannot come about unless the saw has teeth of a certain kind, and these cannot be unless they are of iron. For in the definition too there are some parts that are, as it were, its matter.
269. Having shown that nature acts for an end, the Philosopher here proceeds to inquire into the second question, that is, how necessity is found in natural things.
Concerning this, he makes three points.
First, he raises the question.
Second, at “the current view” (199b35; [271]), he sets forth the opinion of others.
Third, at “nevertheless, though the wall” (200a5; [272]), he determines the truth.
270. Thus, he asks (199b34) whether, in natural things, there is a simple necessity—that is, an absolute necessity—or a necessary by condition or by supposition. In order to understand this, it must be noted that the necessity that depends upon prior causes is an absolute necessity, as is clear from the necessity that depends upon matter. That an animal is corruptible is absolutely necessary. For to be composed of contraries is a consequence of being an animal.
In like manner, that which has necessity from the formal cause is also absolutely necessary. For example, man is rational, or a triangle has three angles equal to two right angles, which is reduced to the definition of triangle.
Similarly, that which has necessity from the efficient cause is absolutely necessary. Thus, because of the motion of the sun, it is necessary that day and night alternate. But that which has necessity from that which is posterior in existence is necessary upon condition, or by supposition.
For example, it might be said that it is necessary that this be if this ought to come to be. Necessity of this kind is from the end and from the form insofar as it is the end of generation. Therefore, to ask whether, in natural things, there is a simple necessity or a necessity by supposition is nothing else than to ask whether necessity is found in natural things from the end or from the matter.
271. Next, at “the current view” (199b35), he gives the opinion of others. He says that some are of the opinion that the generation of natural things arises from an absolute necessity of matter. For example, one might say that a wall or a house is such as it is by the necessity of matter, because heavy things are disposed to move downward and light things to rise above. And, because of this, the heavy and hard stones remain in the foundation, while earth, being lighter, rises above the stones, as is clear in walls constructed of tiles that are made of earth. But the timbers, which are the lightest, are placed at the highest point, at the roof. Thus, they thought that the dispositions of natural things have come to be such as they are from the necessity of matter. For example, it might be said that a man has feet below and hands above because of the heaviness or lightness of humors.
272. Next, at “nevertheless, though the wall” (200a5), he determines the truth.
Concerning this, he makes two points.
First, he shows what sort of necessity there is in natural things;
second, at “necessity in mathematics” (200a15; [273]), he compares the necessity of natural things to the necessity that is in the demonstrative sciences.