Commentary Aristotle Physics
Lecture 11. Natural philosophy demonstrates from all four causes
Now, the causes being four, it is the business of the physicist to know about them all, and if he refers his problems back to all of them, he will assign the “why” in the way proper to his science—the matter, the form, the mover, and “that for the sake of which.”
The last three often coincide, for the “what” and “that for the sake of which” are one, while the primary source of motion is the same in species as these (for man generates man).
And so too, in general, are all things that cause motion by being themselves moved;
and such as are not of this kind are no longer inside the province of physics, for they cause motion not by possessing motion or a source of motion in themselves, but being themselves incapable of motion.
Hence there are three branches of study: the first of things that are incapable of motion; the second of things in motion but indestructible; the third of destructible things.
The question of why, then, is answered by reference to the matter, to what the thing is, and to the primary moving cause.
For in respect of generation, it is mostly in this last way that causes are investigated—“What comes to be after what? What is it that first makes, or undergoes change?” and so at each step of the series.
Now, the principles that cause motion in a physical way are two, of which one is not physical, as it has no principle of motion in itself. Of this kind is whatever causes motion but is not itself moved, such as that which is completely unchangeable, the primary reality.
Of this kind is also the essence of that which is coming to be, namely, the form. For this is the end, or “that for the sake of which.”
Hence, since nature is for the sake of something, we must know this cause also.
We must explain the “why” fully: one, that “from this” that will necessarily result (“from this” either simply or most of the time);
and “this must be so if that is to be so” (as the conclusion presupposes the premises);
and that this was the essence of the thing;
and because it is better thus (not without qualification, but with reference to the essential nature in each case).
241. Having treated the causes, the Philosopher here shows that the natural philosopher demonstrates from all the causes.
Concerning this, he makes two points.
First, he states his intention.
Second, at “the last three” (198a24; [242]), he explains his position.
He therefore says first (198a22) that, inasmuch as there are four causes, as was said above, it pertains to natural science both to know all of them and to demonstrate naturally through all of them by reducing the question “why?” to each of the aforementioned causes: the form, the moving cause, the end, and the matter.
Next, at “the last three” (198a24; [242]), he explains his position.
Concerning this, he makes two points.
First, he sets forth certain things that are necessary to clarify his position.
Second, at “the question” (198a31; [244]), he proves his position.
Concerning the first point, he sets forth two things that are necessary for the proof of what follows.
The first of these deals with the relationship of the causes among themselves.
The second deals with the consideration of natural philosophy and is given at “and so too, in general” (198a27; [243]).
242. He says first (198a24) that it often happens that three of the causes combine into one, such that the formal cause and the final cause are one in number. This must be understood to apply to the final cause of generation but not to the final cause of the thing generated. For the end of the generation of man is the human form, but this form is not the end of man. Rather, man acts for his end through this form. But the moving cause is the same as both of these according to species. And this is especially true in univocal agents, in which the agent produces something like unto itself according to species, as man generates man. For in these cases, the form of the generator, which is the principle of generation, is the same in species as the form of the generated, which is the end of the generation.
However, in non-univocal agents, the species is different. For in these cases, the things that come to be cannot reach the point where they follow upon the form of the generator according to the same kind of species. Rather, they participate in some likeness to it insofar as they are able, as is clear in those things that are generated by the sun. Therefore, the agent is not always the same in species with the form, which is the end of generation; furthermore, not every end is a form. And because of this, it is significant that he said often. But the matter is neither the same in species nor the same in number as the other causes. For matter as such is being in potency, whereas the agent as such is being in act, and the form or the end is act or perfection.
243. Next, at “and so too” (198a27), he makes his second point that deals with the things that natural philosophy should treat. He says that it pertains to natural philosophy to consider any movers that move in such a way that they are moved. But things that move but are not themselves moved do not belong within the consideration of natural philosophy, which properly considers natural things that have in themselves a principle of motion. For movers that are not themselves moved do not have in themselves a principle of motion, since they are not moved, but are immobile. Thus, they are not natural things, and as a result do not come under the consideration of natural philosophy.
Hence, it is clear that there are three branches of study; the study and intention of philosophy is threefold according to the three genera of things which are found.
For some things are immobile, and one philosophical study deals with them.
Another philosophical study deals with things that are mobile but incorruptible, such as the celestial bodies.
And there is a third philosophical study that deals with things that are mobile and corruptible, such as the inferior bodies.
The first of these studies pertains to metaphysics, while the other two pertain to natural science, which treats all mobile things, both corruptible and incorruptible.
Hence some have misunderstood this passage, desiring to reduce these three studies to the three parts of philosophy: mathematics, metaphysics, and physics. For astronomy, which seems to consider the incorruptible mobile things, belongs more to natural philosophy than to mathematics, as was said above. For insofar as it applies mathematical principles to natural matter, it considers mobile things. Therefore, this division is taken according to the diversity of things existing outside the mind, and not according to the division of the sciences.
244. Next, at “the question of why” (198a31), he sets forth his position.
Concerning this, he makes two points.
First, he shows that it pertains to natural philosophy to consider all the causes and to demonstrate through them. These are the two points he has proposed above.
Second, at “we must explain, then” (198b10; [250]), he proves certain things that are assumed in this argument.
Concerning the first part, he makes two points.
First, he shows that natural philosophy considers all the causes.
Second, at we must explain the “why” (198b5; [247]), he shows that it demonstrates through all of them.
Concerning the first part, he makes two points.
First, he shows that natural philosophy considers the matter and the form and the moving cause.
Second, at “of this kind is also” (198b3; [246]), he shows that it considers the end.
Concerning the first part, he makes two points.
First, he states his intention;
second, he proves it, at “for in respect of” (198a33; [245]).
First (198a31) he concludes from what was said above that the “why” is assigned to natural things by reference to the matter, and to what the thing is (namely, the form), and to the first mover.
245. Next, at “for in respect of” (198a33), he proves his position as follows. It has been said that natural philosophy considers those things that are moved, both the generable and the corruptible. Therefore, whatever should be considered about generation should be considered by natural philosophy. But, with reference to generation, one ought to consider the form, the matter, and the moving cause.
Those who wish to consider the causes of generation consider them as follows.
First, we consider what it is that comes to be after something, as fire comes to be after air, since fire is generated from air. And the form, through which the generated is what it is, is considered in this way.
Next, we consider what it is that first makes; that is, we consider that which first moves to generation. And this is the moving cause.
Next, we consider what it is that undergoes this change. And this is the subject and the matter. With reference to generation, we consider not only the first mover and the first subject, but also those things that are consequent upon them.
And thus it is clear that it pertains to natural philosophy to consider the form, the mover, and the matter. However, natural philosophy does not consider every mover. For there are two kinds of moving principles: the moved and the unmoved. Now, a mover that is not moved is not natural, because it does not have in itself a principle of motion. And such is the moving principle that is altogether immobile and the first of all movers, as will be shown in book 8.
246. Next, at “of this kind is also” (198b3), he shows that natural philosophy also considers the end. He says that the form and what the thing is also fall under the consideration of natural philosophy, insofar as the end is that for the sake of which the generation occurs. For it was said above that the form and the end coincide in the same thing. And since nature acts for the sake of something, as will be proven below, it must belong to natural philosophy to consider the form not only insofar as it is form but also insofar as it is the end. If, however, nature were not to act for the sake of something, then natural philosophy would consider form insofar as it is form, but not insofar as it is an end.
247. Next, at we must explain the “why” (198b5), he shows how natural philosophy demonstrates through all the causes.
First, he shows how it demonstrates through matter and the moving cause, which are the prior causes in generation.
Second, at that “this must be so” (198b7; [248]), he shows how it demonstrates through the form.
Third, at “and because it is better” (198b9; [249]), he shows how it demonstrates through the end.
He says first (198b5) that, in natural things, the “why” must be elaborated fully, that is, in every genus of cause. Thus, if something has gone before—whether it be the matter or the mover—then something necessarily follows. For example, if something is generated from contraries, it is necessary that the latter be corrupted, and if the sun approaches the north pole, the days must become longer, and cold must diminish and heat increase for those who dwell in the northern part. However, we must realize that it is not always necessary that something follows from a preceding matter or mover. Rather, sometimes a thing follows simply or in every case, as in the things mentioned. But sometimes a thing follows in most instances: for instance, from human seed and a mover in generation, it follows in most instances that what is generated has two eyes, but at times this fails to happen. Similarly, because of the fact that matter is so disposed in the human body, it happens that a fever is frequently produced because of festering, but at times this is impeded.
248. Next, at and “this must be so” (198b7), he shows how, in natural things, demonstration must be made through the formal cause. In order to understand this, we must know that, when something follows from the preceding causes in generation (that is, from the matter and the mover) by necessity, then a demonstration can be established, as was said above.
However, a demonstration cannot be established when something follows in most instances. But, then, a demonstration must be taken from what is posterior in generation so that something should follow necessarily from another, as the conclusion follows from the principles of a demonstration.
Let us proceed thus in demonstration: if this should come to be, then this and that are required. For example, if man should be generated, it is necessary that human seed be an agent in the generation.
If, however, we proceed conversely by saying that human seed is an agent in generation, then the proposition that therefore, man will be generated does not follow as a conclusion follows from propositions.
But that which ought to come to be—namely, that in which the generation is terminated—was, as was said above, what the thing was to be, namely, the form.
Hence, it is clear that, when we demonstrate according to this mode—namely, that this must be so if that is to be so—we demonstrate through the formal cause.
249. Next, at “and because it is better” (198b9), he shows how natural philosophy demonstrates through the final cause. He says that natural philosophy sometimes also demonstrates that something is true because it is better that it be so. For example, we might demonstrate that the front teeth are sharp because, as such, they are better for cutting food, and nature does what is better. Nature does not, however, do what is better simply, but what is better with reference to what belongs to each substance; otherwise, nature would give a rational soul, which is better than an irrational soul, to each animal.