Lecture 2 — Principles of the natural changes to be considered in this book; their relations to each other

After this introduction let us begin by discussing our immediate subject.

We have already laid down that there is one physical element which makes up the system of the bodies that move in a circle, and besides this four bodies owing their existence to the four principles, the motion of these latter bodies being of two kinds: either from the centre or to the centre. These four bodies are fire, air, water, earth. Fire occupies the highest place among them all, earth the lowest, and two elements correspond to these in their relation to one another, air being nearest to fire, water to earth. The whole world surrounding the earth, then, the affections of which are our subject, is made up of these bodies.

This world necessarily has a certain continuity with the upper motions: consequently all its power and order is derived from them. (For the originating principle of all motion is the first cause.

Besides, that element is eternal and its motion has no limit in space, but is always complete; whereas all these other bodies have separate regions which limit one another.) So we must treat fire and earth and the elements like them as the material causes of the events in this world (meaning by material what is subject and is affected), but must assign causality in the sense of the originating principle of motion to the influence of the eternally moving bodies.

10. Having completed an introduction, in which the Philosopher has revealed his intention, he now begins to show his proposition. And this is divided into two parts:

in the first he restates facts necessary for knowing the principles of the transmutations to be treated in this book;

in the second part he begins to treat of them, at let us first recall (339a33).

About the first he does two things:

first, he enumerates the principles of these transmutations and their difference from one another;

second, he shows how they are related to one another in causing, at this world necessarily has a certain continuity (339a21).

11. He says therefore first that it has been previously determined both in the book, On the Heavens, and in the book, On Generation, that among the other corporeal principles that are principles of other bodies there is one which is the principle of those bodies from which is constituted the nature of the bodies circularly moved, i.e., of spheres and stars. This principle, out of which all such bodies are formed, he calls the fifth essence. The other principles, of the lower bodies, are four in number, because of the primary tangible qualities, which are the principles of acting and of being acted upon. These are the hot, cold, moist and dry, of which there are but four possible combinations: for the hot and dry is fire, the hot and moist is air, the cold and moist is water, and the cold and dry is earth (that something should be hot and cold, or moist and dry, is impossible).

Of these four bodies there are two motions: one is upward from the middle [center] of the world, and this is the motion of light things, namely, fire and air; the other is to the middle [center], and this is the motion of heavy things, namely, earth and water.

Accordingly, bodily motions are threefold: namely, to the middle for heavy bodies; from the middle for light bodies; and about the middle for the heavenly bodies, which are neither heavy nor light. But notice should be taken of the differences in light and heavy. For there is something which is absolutely light, namely, fire, which is above all the others; there is something which is absolutely heavy, namely, earth, which is under all the others. But the other two are in a certain respect heavy and light—for air is light in relation to earth and water, but heavy in relation to fire; water, on the other hand, is light with respect to earth, but heavy with respect to air and fire.

Consequently, these two are proportional to the other two that are extremes, i.e., as air is nearer to fire, so water is nearer to earth. Thus it is plain that the universe around the earth consists of four bodies. This is the world chose passions—which are the various transmutations found in the elements—we must consider in this book.

12. Then when he says, this world necessarily has a certain continuity (339a21), he shows how the aforesaid principles are related to one another in causing. And he says that it is necessary for the lower world to consist of the four elements thus in continuity with the upper movements, i.e., with the bodies circularly moved (by continuous he here means "contiguous," in the sense that nothing lies between them). The reason why this is necessary is not only because no empty place can exist (hence bodies must be contiguous to bodies), but also because the end requires it—the end being that the whole power of the lower world be governed by the superior bodies, and this would not be, unless they touched—for a bodily agent must touch the thing acted upon and moved by it.

13. That the lower world is ruled and moved by the superior bodies he proves with two arguments. The first is this: The movent cause, i.e., the originative principle of motion, is necessarily the first cause. (This is to be understood in its relation to the formal and material cause. For matter is acted upon by the agent, which is by nature prior to the patient. The form, too, is an effect of the movent, which educes matter from potency to act. But the end is prior to the agent, because it moves the agent. Yet it is not always prior in the order of existence, but [sometimes] only in the order of intention.) Now it is plain that in the sphere of natural things the heavenly body is the first cause, and this is proved from its incorruptibility and nobility. Consequently, the heavenly body, with respect to these lower bodies, must be the originative cause of motion.

14. He gives the second argument at besides, that element (339a24), which is this: The motion of the heavenly body is perpetual. This is apparent from the very disposition of place: for in the case of a straight line, one arrives at an end in act, namely, the terminus of the line, but in the case of a circle, one does not arrive at an end. He says, therefore, that circular motion does not have an end according to place. And lest anyone conclude from this that circular motion is imperfect, on the same ground that a straight motion is imperfect before reaching its end, he adds that a circular motion is always at an end—for any point you designate on a circle is both a beginning and an end, and circular motion is as perfect at any sector as a straight motion is at its end.

Therefore it is plain, from the very disposition of place, that, to heavenly motions, perpetuity is congruent. The motions of lower bodies, on the other hand, cannot be perpetual, because such bodies are moved with rectilinear motions, and a rectilinear motion remains one and continuous only according to the measure of the rectilinear magnitude along which the motion passes, and a reflected motion is not continuous, as was proved in Physics VIII.

Hence, since all the lower bodies are a finite distance from one another, and no such body is infinite, as was proved in Physics III and in On the Heavens I, their motions must be finite and not perpetual. What is perpetual and always, consequently, is the movent of those things which are not always.

Wherefore the lower elements, namely, fire and earth and the others which are syngeneous, i.e., congenerable, with them, namely, air and water, and those things which are composed out of them, must be reckoned the causes of the things occurring in the lower world, in the line of matter, i.e., after the manner of the material cause, because that is the way we speak of a subject and a patient being a cause of things. But their cause in the sense of originative source of motion, i.e., their cause after the manner of movent cause, must be sustained, i.e., held, to be that power which belongs to the always moved, i.e., to the heavenly bodies, which are always in motion—for what is always in motion is compared to what is not forever in motion as agent is compared to patient.