From this point Aristotle goes on (Phys. IV. 6, 7) to empty space, in which an old question is involved which physicists even now cannot explain: they could do so if they studied Aristotle, but as far as they are concerned there might have been no thought nor Aristotle in the world. “Vacuum, according to ordinary ideas, is a space in which there is no body, and, fancying that all Being is body, they say that vacuum is that in which there is nothing at all. The conception of a vacuum has its justification for one thing in the fact that a vacuum,” the negative to an existent form, “is essential to motion; for a body could not move in a plenum,” and in the place to which it does move there must be nothing. “The other argument in favour of a vacuum is found in the compression of bodies, in which the parts press into the empty spaces.” This is the conception of varying density and the alteration of the same, in accordance with which an equal weight might consist of an equal number of parts, but these, as being separated by vacuum, might present a greater volume. Aristotle confutes these reasonings most adroitly, and first of all in this way; “The plenum could be changed, and bodies could yield to one another even if no interval of vacuum separated them. Liquids as well as solids are not condensed into a vacuum; something that they contained is expelled, just as air is expelled if water is compressed.”
Aristotle deals more thoroughly, in the first place (Phys. IV. 8), with the erroneous conception that the vacuum is the cause of movement. For, on the one hand, he shows that the vacuum really abolishes motion, and consequently in vacuum a universal rest would reign. He calls it perfect indifference as to the greater or less distance to which a thing is moved; in vacuum there are no distinctions. It is pure negation without object or difference; there is no reason for standing still or going on. But body is in movement, and that, indeed, as distinguished; it has a positive relation, and not one merely to nothing. On the other hand, Aristotle refutes the idea that movement is in vacuum because compression is possible. But this does not happen in a vacuum; there would be established in it not one movement, but a movement towards all sides, a general annihilation, an absolute yielding, where no cohesion would remain in the body. “Again, a weight or a body is borne along more swiftly or more slowly from two causes; either because there is a difference in that through which it is borne along, as when it moves through air or water or earth, or because that which is borne along differs through excess of weight or lightness.” As regards difference of movement on account of the first difference—that in the density of the medium—Aristotle says: “The medium through which the body is borne along is the cause of the resistance encountered, which is greater if the medium is moving in a contrary direction (and less if it is at rest); resistance is increased also if the medium is not easily divided. The difference in velocity is in inverse ratio to the specific gravity of the medium, air and water, so that if the medium has only half the density, the rate of progress will be double as quick. But vacuum has to body no such relation of differences of specific gravity. Body can no more contain a vacuum within its dimensions than a line can contain a point, unless the line were composed of points. The vacuum has no ratio to the plenum.” But as to the other case, the difference in weight and lightness, which has to be considered as being in bodies themselves, whereby one moves more quickly than another through the same space: “this distinction exists only in the plenum, for the heavy body, by reason of its power, divides the plenum more quickly.” This point of view is quite correct, and it is mainly directed against a number of conceptions that prevail in our physics. The conception of equal movement of the heavy and the light, as that of pure weight, pure matter, is an abstraction, being taken as though they were inherently like, only differing through the accidental resistance of the air.
Aristotle (Phys. IV. 9) now comes to the second point, to the proof of the vacuum because of the difference in specific gravity. “Many believe that the vacuum exists because of the rare and the dense;” the former is said to be a rare body, and the latter a perfect continuity; or they at least differ quantitatively from one another through greater or less density. “For if air should be generated from a quantity of water, a given quantity of water must produce a quantity of air the same in bulk, or there must necessarily be a vacuum; for it is only on the hypothesis of a vacuum that compression and rarefaction are explicable. Now if, as they say, the less dense were that which has many separate void spaces, it is evident that since a vacuum cannot be separated any more than a space can have intervals, neither can the rare subsist in this manner. But if it is not separable, and yet a vacuum is said to exist in the body, in the first place movement could thus only be upwards; for the rare is the light, and hence they say that fire is rare,” because it always moves upwards. “In the next place the vacuum cannot be the cause of motion as that in which something moves, but must resemble bladders that carry up that which adheres to them. But how is it possible that a vacuum can move, or that there can be a place where there is a vacuum? For that into which it is carried would be the vacuum of a vacuum. In short, as there can be no movement in vacuum, so also a vacuum cannot move.” Aristotle set against these ideas the true state of matters, and states generally the ideal conception of nature: “that the opposites, hot and cold, and the other physical contraries, have one and the same matter, and that from what is in capacity that which is in energy is produced; that matter is not separable though it is different in essence[94] (τῷ εἶναι), and that it remains one and the same in number (ἀριθμῷ) even if it possesses colour, or is hot and cold. And again, the matter of a small body and a large is the same, because at one time a greater proceeds from a smaller, and at another time a smaller from a greater. If air is generated from water it is expanded, but the matter remains the same and without taking to itself anything else; for that which it was in capacity it becomes in actuality. In a similar way if air is compressed from a greater into a less volume, the process will be reversed, and air will similarly pass into water, because the matter which is in capacity both air and water, also becomes both.” Aristotle likewise asserts that increase and decrease of warmth, and its transition into cold, is no addition or otherwise of warm matter, and also one and the same is both dense and rare. This is very different from the physical conceptions which assert more or less matter to correspond with more or less density, thus comprehending the difference in specific weight as the external addition of matter. Aristotle, on the contrary, takes this dynamically, though certainly not in the sense in which dynamics are to-day understood, viz. as an increase of intensity or as a degree, for he accepts intensity in its truth as universal capacity. Undoubtedly the difference must also be taken as a difference in amount, but not as an increase and decrease, or as an alteration in the absolute quantity of the matter. For here intensity means force, but again not as being a thing of thought separated from matter, but as indicating that if anything has become more intensive, it has had its actuality diminished, having, however, according to Aristotle, attained to a greater capacity. If the intensity is again directed outwards, and compared with other things, it undoubtedly becomes degree, and therefore magnitude immediately enters in. It then is indifferent whether greater intension or greater extension is posited; more air is capable of being warmed to the same degree as less, through the greater intensity of the warmth; or the same air can thereby become intensively warmer.
As regards the investigation of time, Aristotle remarks (Phys. IV. 10, 11, 13) that if time is externally (exoterically, ἐξωτερικῶς) regarded, we are inevitably led to doubt (διαπορῆσαι) whether it has any being whatever, or whether it has bare existence, as feeble (μόλις καὶ ἀμυδρῶς) as if it were only a potentiality. “For one part of it was and is not: another part will be and is not as yet; but of these parts infinite and everlasting (ἀεὶ λαμβανόμενος), time is composed. But it now appears that time, if composed of things that are not, may be incapable of existence. And also as regards everything divisible, if it exists, either some or all of its parts must be. Time is certainly divisible; but some of the parts are past, others are future, and no part is present. For the now is no part, since a part has a measure, and the whole must consist of the parts; but time does not appear to consist of the Now.” That is to say, because the Now is indivisible, it has no quantitative determination which could be measured. “Besides it is not easy to decide whether the Now remains, or always becomes another and another. Again, time is not a movement and change, for movement and change occur in that which is moved and changed, or accompany time in its course; but time is everywhere alike. Besides change is swifter and slower, but time is not. But it is not without change and motion” (which is just the moment of pure negativity in the same) “for when we perceive no change, it appears as if no time had elapsed, as in sleep. Time is hence in motion but not motion itself.” Aristotle defines it thus: “We say that time is, when we perceive the before and after in movement; but these are so distinguished that we apprehend them to be another and another, and conceive that there is something between, as a middle. Now when we understand that the extremes of the conclusion are different from the middle, and the soul says that the Now has two instants, the one prior and the other posterior, then we say that this is time. What is determined through the Now, we call time, and this is the fundamental principle. But when we are sensible of the Now as one, and not as a prior and posterior in motion, nor as the identity of an earlier or later, then there does not appear to us to have been any time, because neither was there any motion.” Tedium is thus ever the same. “Time is hence the number of motion, according to priority and posteriority; it is not motion itself, unless so far as motion has number. We judge of the more or less through number, but of a greater or less motion by time. But we call number that which can be numbered, as well as that with which we number; but time is not the number with which we number, but that which is numbered, and, like motion, always is changing. The Now is, which is the unity of number, and it measures time. The whole of time is the same, for the Now which was is the same” (universality as the Now destroyed) “but in Being it is another. Time thus is through the Now both continuous (συνεχής) and discrete (διῇρηται). It thereby resembles the point, for that also is the continuity of the line and its division, its principle and limit; but the Now is not an enduring point. As continuity of time the Now connects the past and the future, but it likewise divides time in capacity,” the Now is only divisibility and the moments only ideal. “And in as far as it is such, it is always another; but, in as far as it unites, it is ever one and the same. Similarly, in as far as we divide the line, other and yet other points always arise for thought; but in as far as it is one, there is only one point. Thus the Now is both the division of time in capacity, and the limit and union of both” i.e. of the prior and posterior. The universally dividing point is only one as actual; but this actual is not permanently one, but ever and again another, so that individuality has universality, as its negativity, within it. “But division and union are the same, and similarly related; however their Notion (τὸ εἶναι)[95] is different.” In one and the same respect the absolute opposite of what was posited is immediately set forth as existent; in space, on the other hand, the moments are not set forth as existent, but in it first appears this being and its motion and contradiction. Thus the identity of the understanding is not a principle with Aristotle, for identity and non-identity to him are one and the same. Because the Now is only now, past and future are different from it, but they are likewise necessarily connected in the Now, which is not without before and after; thus they are in one, and the Now, as their limit, is both their union and their division.
Aristotle (Phys. V. 1) then goes on to movement as realized in a thing, to change (μεταβολή) or to the physical processes—while before we had pure movement. “In movement there is first something which moves, also something which is moved, and the time in which it is moved; besides these, that from which, and that into which it is moved.” (Cf. supra, p. 141.) “For all motion is from something and into something; but there is a difference between that which is first moved and that into which and from which it is moved, as, for instance, wood, warmth and cold. The motion is in the wood and not in the form; for neither form nor place, nor quantity moves or is moved, but” (in the order in which they follow) “there is that which is moved and that which moves, and that into which it is moved. That to which movement is made, more than that out of which movement is made, is named change. Hence to pass into non-being is also change, although what passes away is changed from Being: and generation is a mutation into Being, even though it is from non-being.” The remark is to be interpreted as meaning that for the first time in real becoming motion, i.e. in change, the relation whereto enters, while the relation wherefrom is that in which change is still the mere ideal motion. Besides this first form of difference between motion and change, Aristotle further gives another, since he divides change into three: “into change from a subject (ἐξ ὑποκειμένον) into a subject; or from a subject into a non-subject; or from a non-subject into a subject.” The fourth, “from a non-subject into a non-subject,” which may also appear in the general division, “is no mutation, for it contains no opposition.” It may certainly be merely thought or ideal, but Aristotle indicates the actual phenomenon. “The mutation from a non-subject into a subject is generation (γένεσις); that from a subject into a non-subject is corruption (φθορά); that from a subject into a subject, is motion as such;” because that which is transformed remains the same, there is no becoming-another of the actual, but a merely formal becoming-another. This opposition of the materialized motion as mutation, and of merely formal motion, is noteworthy.
In the sixth book Aristotle comes to the consideration of the dialectic of this motion and change as advanced by Zeno, that is, to the endless divisibility which we have already (Vol. I. pp. 266-277) considered. Aristotle solves it through the universal. He says that they are the contradiction of the universal turned against itself; the unity in which its moments dissolve is not a nothing, so that motion and change are nothing, but a negative universal, where the negative is itself again posited as positive, and that is the essence of divisibility.
Of the further details into which Aristotle enters, I shall only give the following. As against atoms and their motion, he remarks (Phys. VI. 10) that the indivisible has no motion and mutation, which is the direct opposite of the proposition of Zeno that only simple indivisible Being and no motion exists. For as Zeno argues from the indivisibility of atoms against motion, Aristotle argues from motion against atoms. “Everything which moves or changes is in the first division of this time partly here and partly there. The atom, as simple indivisible Being, can, however, not have any part of it in both points in space, because it then would be divisible. The indivisible could thus only move if time consisted of the Now; this is, however, impossible, as we proved before.” Because atoms thus neither have change in themselves, nor can this come to them from without through impulse, &c., they are really without truth.
The determination of the pure ideality of change is important. Aristotle says of this (Phys. VII. 3), “That which is changed is alone the sensuous and perceptible (αἰσθητόν); and forms and figures, as also capacities, are not changed, they arise and disappear in a thing only, without being themselves changed.” In other words: the content of change is unchangeable; change as such belongs to mere form. “Virtues or vices belong, for example, to habits acquired. Virtue is the perfection (τελείωσις) in which something has reached the end of its nature. Vice, however, is the corruption and non-attainment of this. They are not changes, for they only arise and pass away while another alters.” Or the difference becomes a difference of Being and non-being, i.e. a merely sensuous difference.
From these conceptions Aristotle now comes nearer to the first real or physical motion (Phys. VIII. 6, 8, 9; De C\nlo, I. 4): The first principle of motion is itself unmoved. An endless motion in a straight line is an empty creation of thought; for motion is necessarily an effort after something. The absolute motion is the circular, because it is without opposition. For because movement has to be considered in regard to the starting-place and the end in view, in the straight movement the directions from A to B and from B to A are opposed, but in motion in a circle they are the same. The idea that heavenly bodies would of themselves have moved in a straight line, but that they accidentally came into the sphere of solar attraction, is an empty reflection which is far from occurring to Aristotle.