by Aristotle
No absurdity is involved in the fact that something not in motion may be set in motion, that which is to cause motion from without being at one time present, and at another absent. Nevertheless, how this can be so remains matter for inquiry; how it [5] comes about, I mean, that the same motive force at one time causes a thing to be in motion, and at another does not do so; for the difficulty raised by our objector really amounts to this—why is it that some things are not always at rest, and others always in motion?
The third objection may be thought to present more difficulty than the others, namely, that which alleges that motion arises in things in which it did not exist before, and adduces in proof the case of animate things: thus an animal is first at [10] rest and afterwards walks, not having been set in motion apparently by anything from without. This, however, is false; for we observe that there is always some part of the animal’s organism in motion, and the cause of the motion of this part is not the animal itself, but, it may be, its environment. Moreover, we say that the animal itself originates not all of its motions but its locomotion. So it may well be the [15] case—or rather perhaps it must be the case—that many motions are produced in the body by its environment, and some of these set in motion the intellect or the appetite, and this again then sets the whole animal in motion: this is what happens in sleep: though there is then no perceptive motion in them, there is some motion that causes them to wake up again. But we will leave this point also to be elucidated [20] at a later stage in our discussion.
3 · Our enquiry will resolve itself at the outset into a consideration of the above-mentioned problem—what can be the reason why some things in the world at one time are in motion and at another are at rest again? Now one of three things must be true: either all things are always at rest, or all things are always in motion, or some things are in motion and others at rest; and in this last case again either the [25] things that are in motion are always in motion and the things that are at rest are always at rest, or they are all naturally capable alike of motion and of rest; or there is yet a third possibility remaining—it may be that some things in the world are always motionless, others always in motion, while others again admit of both conditions. This last is the account of the matter that we must give; for herein lies [30] the solution of all the difficulties raised and the conclusion of the investigation upon which we are engaged.
To maintain that all things are at rest, and to disregard sense-perception and attempt to show the theory to be reasonable, would be an instance of intellectual weakness: it would call in question a whole system, not a particular detail; moreover, it would be an attack not only on the physicist but on almost all sciences [253b1] and all opinions, since motion plays a part in all of them. Further, just as in arguments about mathematics objections that involve first principles do not affect the mathematician—and the other sciences are in similar case—so, too, objections involving the point that we have just raised do not affect the physicist; for it is a [5] hypothesis that nature is a principle of motion.
The assertion that all things are in motion we may fairly regard as false, though it is less subversive of physical science; for though in our course on physics it was laid down that nature is a principle of rest no less than of motion, nevertheless motion is the natural state; moreover, the view is actually held by some that not [10] merely some things but all things in the world are in motion and always in motion, though we cannot apprehend the fact by sense-perception. Although the supporters of this theory do not state clearly what kind of motion they mean, or whether they mean all kinds, it is no hard matter to reply to them. For there cannot be a continuous process either of increase or of decrease: that which comes between the two has to be included. The theory resembles that about the stone being worn away by the drop of water or split by plants growing out of it: if so much has been [15] extruded or removed by the drop, it does not follow that half the amount has previously been extruded or removed in half the time; but, as in the case of the hauled ship, so many drops set so much in motion, but a part of them will not set as much in motion in any period of time. The amount removed is, it is true, divisible into a number of parts, but no one of these was set in motion separately: they were [20] all set in motion together. It is evident, then, that from the fact that the decrease is divisible into an infinite number of parts it does not follow that some part must always be passing away: it all passes away at a particular moment. Similarly, too, in the case of any alteration whatever, if that which suffers alteration is infinitely divisible it does not follow from this that the same is true of the alteration itself, [25] which often occurs all at once, as in freezing. Again, when any one has fallen ill, there must follow a period of time in which he will recover: the change cannot take place in an instant; and the change cannot be a change to anything else but health. The assertion, therefore, that alteration is continuous is too much at odds with the [30] evident facts; for alteration is from one contrary to another. Moreover, a stone becomes neither harder nor softer. Again, in the matter of locomotion, it would be a strange thing if a stone could be falling or resting on the ground without our being able to perceive the fact. Again, earth and all other bodies necessarily remain in their proper places and are moved from them only by violence; from the fact, then, that some of them are in their proper places it follows that in respect of place all things cannot be in motion. These and other similar arguments, then, should [254a] convince us that it is impossible either that all things are always in motion or that all things are always at rest.
Nor again can it be that some things are always at rest, others always in motion, and nothing sometimes at rest and sometimes in motion. This theory must [5] be pronounced impossible on the same grounds as those previously mentioned: viz. that we see the above-mentioned changes occurring in the case of the same things. We may further point out that the defender of this position is fighting against the evident facts; for there can be no increase and no compulsory motion, if it is impossible that a thing can be at rest before being set in motion unnaturally. This [10] theory, then, does away with becoming and perishing. Moreover, motion, it would seem, is generally thought to be a sort of becoming and perishing; for a thing comes to be that, or in that, to which it changes; and it ceases to be that, or in that, from which it changes. It is clear, therefore, that there are cases of occasional motion and occasional rest.
[15] We have now to take the assertion that all things are sometimes at rest and sometimes in motion and to confront it with the arguments previously advanced. We must take our start again, as we did before, from the possibilities that we distinguished just above. Either all things are at rest, or all things are in motion, or some things are at rest and others in motion. And if some things are at rest and [20] others in motion, then it must be that either all things are sometimes at rest and sometimes in motion, or some things are always at rest and the remainder always in motion, or some of the things are always at rest and others always in motion while others again are sometimes at rest and sometimes in motion. Now we have said before that it is impossible that all things should be at rest: nevertheless we may now [25] repeat the point. For even if it is really the case, as some assert, that what is is infinite and motionless, it certainly does not appear to be so if we follow sense-perception: many things that exist appear to be in motion. Now if there is such a thing as false opinion or opinion at all, there is also motion; and similarly if there is such a thing as imagination, or if it is the case that anything seems to be [30] different at different times; for imagination and opinion are thought to be motions of a kind. But to investigate this question at all—to seek an argument in a case where we are too well off to require argument—implies bad judgement of what is better and what is worse, what commends itself to belief and what does not, what is a principle and what is not. It is likewise impossible that all things should be in motion or that some things should be always in motion and the remainder always at rest. We have sufficient ground for rejecting all these
theories in the single fact that [254b1] we see some things sometimes in motion and sometimes at rest. It is evident, therefore, that it is no less impossible that some things should be always in motion and the remainder always at rest than that all things should be at rest or that all things should be in motion continuously. It remains, then, to consider whether all things are so constituted as to be capable both of being in motion and of being at rest, or whether, while some things are so constituted, some are always at rest and [5] some are always in motion; for it is this last view that we have to show to be true.
4 · Now of things that cause motion or suffer motion, some do so accidentally, others in their own right—accidentally if they merely belong to or contain as a part a thing that causes motion or suffers motion, in their own right if they cause [10] motion or suffer motion not merely by belonging to such a thing or containing it as a part.
Of things which move in their own right, some derive their motion from themselves, others from something else: and in some cases their motion is natural, in others violent and unnatural. Thus in things that derive their motion from themselves, e.g. all animals, the motion is natural. (For when an animal is in motion [15] its motion is derived from itself; and whenever the source of the motion of a thing is in the thing itself we say that the motion of that thing is natural. Therefore the animal as a whole moves itself naturally; but the body of the animal may be in motion unnaturally as well as naturally: it depends upon the kind of motion that it may chance to be suffering and the kind of element of which it is composed.) And [20] the motion of things that derive their motion from something else is in some cases natural, in others unnatural: e.g. upward motion of earthy things and downward motion of fire are unnatural. Moreover the parts of animals are often in motion in an unnatural way, their positions and the character of the motion being abnormal. The fact that a thing that is in motion derives its motion from something is most [25] evident in things that are in motion unnaturally, because in such cases it is clear that the motion is derived from something other than the thing itself. Next to things that are in motion unnaturally those whose motion while natural is derived from themselves—e.g. animals—make this fact clear; for here the uncertainty is not as to whether the motion is derived from something but as to how we ought to distinguish in the thing between the mover and the moved. It would seem that in animals, just as in ships and things not naturally constituted, that which causes motion is [30] separate from that which suffers motion, and that in this way the animal as a whole causes its own motion.
The greatest difficulty, however, is presented by the remaining case of those that we last distinguished. Where things derive their motion from something else, we laid it down that some move contrary to nature: the others remain to be [255a1] contrasted with them, as moving by nature. It is in these cases that difficulty would be experienced in deciding whence the motion is derived, e.g. in the case of light and heavy things. When these things are in motion to positions the reverse of those they would properly occupy, their motion is violent: when they are in motion to their proper positions—the light thing up and the heavy thing down—their motion is [5] natural; but in this case it is no longer evident, as it is when the motion is unnatural, whence their motion is derived. It is impossible to say that their motion is derived from themselves: this is a characteristic of life and peculiar to living things. Further, if it were, it would have been in their power to stop themselves (I mean that if e.g. a thing can cause itself to walk it can also cause itself not to walk), and so, if fire itself possesses the power of upward locomotion, it is clear that it should also possess the [10] power of downward locomotion. Moreover if things move themselves, it would be unreasonable to suppose that in only one kind of motion is their motion derived from themselves. Again, how can anything continuous and naturally unified move itself? In so far as a thing is one and continuous not merely in virtue of contact, it is impassive: it is only in so far as a thing is divided that one part of it is by nature [15] active and another passive. Therefore none of these things move themselves (for they are naturally unified), nor does anything else that is continuous: in each case the mover must be separate from the moved, as we see to be the case with inanimate things when an animate thing moves them. It is the fact that these things also always derive their motion from something: what it is would become evident if we were to distinguish the different kinds of cause.
[20] The above-mentioned distinctions can also be made in the case of things that cause motion: some of them are capable of causing motion unnaturally (e.g. the lever is not naturally capable of moving the weight), others naturally (e.g. what is actually hot is naturally capable of moving what is potentially hot); and similarly in the case of all other things of this kind.
In the same way, too, what is potentially of a certain quality or of a certain [25] quantity or in a certain place is naturally movable when it contains the corresponding principle in itself and not accidentally (for the same thing may be both of a certain quality and of a certain quantity, but the one is an accidental, not an essential property of the other). So when fire or earth is moved by something the motion is violent when it is unnatural, and natural when it brings to actuality the [30] proper activities that they potentially possess. But the fact that the term ‘potentially’ is used in more than one way is the reason why it is not evident whence such motions as the upward motion of fire and the downward motion of earth are derived. One who is learning a science knows potentially in a different way from one who while already possessing the knowledge is not actually exercising it. Wherever something capable of acting and something capable of being acted on are together, [255b1] what is potential becomes actual: e.g. the learner becomes from one potential something another potential something (for one who possesses knowledge of a science but is not actually exercising it knows the science potentially in a sense, though not in the same sense as before he learnt it). And when he is in this condition, if something does not prevent him, he actively exercises his knowledge: otherwise he would be in the contradictory state of not knowing. In regard to natural bodies also the case is similar. Thus what is cold is potentially hot: then a [5] change takes place and it is fire, and it burns, unless something prevents and hinders it. So, too, with heavy and light: light is generated from heavy, e.g. air from water (for water is first such potentially), and air is actually light, and will at once realize [10] its proper activity unless something prevents it. The activity of lightness consists in the light thing being in a certain place, namely high up: when it is in the contrary place, it is being prevented. The case is similar also in regard to quantity and quality. But, be it noted, this is the question we are trying to answer—how can we account for the motion of light things and heavy things to their proper places? The reason for it is that they have a natural tendency towards a certain position; and this [15] is what it is to be light or heavy, the former being determined by an upward, the latter by a downward, tendency. As we have said, a thing may be potentially light or heavy in more ways than one. Thus not only when a thing is water is it in a sense potentially light, but when it has become air it may be still potentially light; for it may be that through some hindrance it does not occupy an upper position, whereas, [20] if what hinders it is removed, it realizes its activity and continues to rise higher. The process whereby what is of a certain quality changes to a condition of actuality is similar: thus the exercise of knowledge follows at once upon the possession of it unless something prevents it. So, too, what is of a certain quantity extends itself over a certain space unless something prevents it. The thing in a sense is and in a sense is not moved by one who moves what is obstructing and preventing its motion—e.g. one who pulls away a pillar or one who removes the stone from a wineskin in the [25] water is the accidental cause of motion; and in the same way the rebounding ball is moved not by the wall but by the thrower. So it is clear that in all these cases the thing does not move itself, but it contains within itself the source of motion—not
of [30] moving something or of causing motion, but of suffering it.
If then the motion of all things that are in motion is either natural or unnatural and violent, and all things whose motion is violent and unnatural are moved by something, and something other than themselves, and again all things whose motion is natural are moved by something—both those that are moved by themselves and those that are not moved by themselves (e.g. light things and heavy things, which are moved either by that which brought the thing into existence and made it light [256a1] and heavy, or by that which released what was hindering and preventing it); then all things that are in motion must be moved by something.
5 · Now this may come about in either of two ways, either not because of the mover itself, but because of something else which moves the mover, or because of [5] the mover itself. Further, in the latter case, either the mover immediately precedes the last thing in the series, or there may be one or more intermediate links: e.g. the stick moves the stone and is moved by the hand, which again is moved by the man; in the man, however, we have reached a mover that is not so in virtue of being moved by something else. Now we say that the thing is moved both by the last and by the [10] first of the movers, but more strictly by the first, since the first moves the last, whereas the last does not move the first, and the first will move the thing without the last, but the last will not move it without the first: e.g. the stick will not move anything unless it is itself moved by the man. If then everything that is in motion must be moved by something, and by something either moved by something else or [15] not, and in the former case there must be some first mover that is not itself moved by anything else, while in the case of the first mover being of this kind there is no need of another (for it is impossible that there should be an infinite series of movers, each of which is itself moved by something else, since in an infinite series there is no first [20] term)—if then everything that is in motion is moved by something, and the first mover is moved but not by anything else, it must be moved by itself.