Commentary Aristotle Physics
Lecture 5
Things may be moved or at rest in five ways
Our enquiry will resolve itself at the outset into a consideration of the above-mentioned problem: what can be the reason that 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.
In this last case again, either the things that are in motion are always in motion and the things that are at rest are always at rest, or they are all constituted so as to be 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 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 in an attempt to show the theory to be reasonable would be an instance of intellectual weakness.
It would call into question a whole system, not a particular detail; moreover, it would be an attack not only on the physicist, but on almost all sciences 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 similarly in the other sciences—so, too, objections involving the point that we have just raised do not affect the physicist, for it is a fundamental assumption with him that motion is ultimately referable to nature.
The assertion that all things are in motion we may fairly regard as equally false, though it is less subversive of physical science. For though it was laid down in our course on physics that rest no less than motion is ultimately referable to nature herself, nevertheless motion is the characteristic fact of nature.
Moreover, the view is actually held by some that not merely some things in the world are in motion and always in motion, but all things, 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.
Thus we may point out that 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 extruded or removed by the drop, it does not follow that half the amount has previously been extruded or removed in half the time.
The case of the hauled ship is exactly comparable, for here we have so many drops setting 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 all set in motion together.
It is evident 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, 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 his restoration to health is in the future. The process of change cannot take place in an instant, yet the change cannot be a change to anything else but health.
The assertion that alteration is continuous is therefore an extravagant calling into question of the obvious: for alteration is a change from one contrary to another.
Moreover, we notice that 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.
Further, it is a law of nature that earth and all other bodies should remain in their proper places and be moved from them only by violence. Thus, from the fact that some of them are in their proper places, it follows that, in respect of place also, all things cannot be in motion. These and other similar arguments, then, should convince us that it is impossible either that all things are always in motion or that all things are always at rest.
1004. Having shown in book 7 (242a15) that there is not an infinite process in movers and in mobiles, but that a first must be reached, and having now shown that motion has always been and always will be, the Philosopher goes on further to consider the condition of the first motion and of the first mover.
And his treatment is divided into two parts. In the first, he shows that the first motion is eternal and that the first mover is entirely immobile; second, he proceeds from this to show the condition of the first motion and of the first mover, at this matter will be (260a20; [1086]).
The first is divided into three parts: in the first, he gives a division having five members; in the second, he excludes three members of this division, at “to maintain that” (253a32; [1006]); third, he investigates the two remaining members to see which of them is truer, because the truth of what he intends to settle depends on it at, we have now to take (254a15; [1016]).
1005. He says first (253a22), therefore, that the reason for the following consideration, in which we intend to investigate about the first motion and the first mover, is that it pertains to a question he raised in answering the second argument: it may be that some things in the world are always motionless, others always in motion, and are not either always in motion or always at rest, since motion in common is considered perpetual.
And he says that the ways in which things are disposed with respect to motion or rest are necessarily limited to three.
The first of these is that all things be always at rest and nothing ever in motion;
the second is that all things be always in motion and nothing at rest;
the third way is that some things are in motion and others at rest.
But the third way is again divided into three ways.
The first of these is that some things are in motion and some at rest in the sense that the ones in motion are always in motion and those at rest always at rest, and nothing is at one time in motion and at another time at rest.
The second way is the converse, that all things are apt to be in motion and at rest, and that nothing is either always in motion or always at rest.
The third way is that certain things are always immobile and never in motion, others are always mobile and never at rest, and still others may be taken that admit of both, that is, of motion and rest, so as to be in motion at one time and at rest at another.
This last way must be determined by us to be the truth, because it contains the solutions of all the objections. And, when we shall have shown this, we shall possess the end that we intend in this work, namely, to arrive at a first eternal motion and a first immobile mover.
Therefore, it is in the above manner that the third member of the first division is divided into three members, thus making a general division consisting of five members.
Now, it should be noticed that, in three of these members, all things are put in one definite disposition. In the first member, all things are taken to be always at rest; in the second, all things are always in motion; and in the fourth all things alternate between motion and rest. But, in one member, the third, beings are divided according to two dispositions, such that some are always in motion and others always at rest. Finally, in the fifth, beings are divided according to three dispositions: some are never in motion, others are never at rest, and yet others are sometimes in motion and sometimes not. Notice, too, that, in this last member, it is not rest that is posited, but immobility; because the first mover, who is never moved, cannot strictly be said to be at rest, for as was said in book 5, only what is apt to be moved but is not being moved is properly said to be at rest.
1006. Then, at “to maintain that” (253a32), he excludes three members of the division.
First, he posits that not all things are always at rest;
second, that not all things are always in motion, at “the assertion that” (253b6; [1007]);
third, he excludes the third member, in which it was said that the things in motion are always in motion and those at rest are always at rest, at “nor again can it be” (254a3; [1014]).
In regard to the first, he posits three things.
The first of these is that it is due to a weakness of understanding that some maintain that all things are at rest and, in support of their stand, search for a sophistic reason without referring to sense. For it proceeds from the fact that the intellect is not capable of destroying sophistical arguments that conflict with things evident to sense. But it has been said in Topics 1.9 that there is no need to dispute against positions or problems that are in a mind that needs sense or punishment. Hence, it is not necessary to dispute this position, due to its stupidity.
The second thing he says is that this problem does not concern a particular being, but being in general. Nor does it affect natural science alone, but in a way, all demonstrative sciences and all opinions, that is, all the arts that use opinions, as do rhetoric and dialectics, for all the arts and sciences make use of motion. For the practical arts, in a way, direct certain motions, and natural philosophy speculates about the nature of motion and about mobile beings. Mathematicians, too, make use of motion, namely, of an imagined one, saying that a point in motion makes a line. The metaphysician, however, considers first principles. Accordingly, it is plain that to destroy motion conflicts with all sciences. Now, an error that affects all beings and all sciences is not to be reproved by the philosopher of nature, but by the metaphysician. Therefore, it is not the business of natural philosophy to dispute this error.
The third thing he says is that unreasonable and impertinent problems about the principles of mathematical sciences do not pertain to mathematics to be answered. The same is true in the other sciences. In like manner, it is not the business of the physicist to destroy an affirmation that is contrary to its principles. For in each science, the definition of the subject is assumed as a principle; hence, in the science that deals with nature, it is assumed as a principle that nature is a principle of motion.
Accordingly, in the light of these three statements, it is apparent that it does not belong to natural philosophy to dispute this position.
1007. Then, at “the assertion that” (253b6), he excludes the second member, in which Heraclitus posited that all things are always being moved.
He first compares this opinion with the previous one that posited that all things are always at rest; and he says that saying that all things are always in motion, as Heraclitus did, is both false and contrary to the principles of natural science. Yet this position is not in as great conflict with the art as the first one is.
But that it does conflict with the art is clear, because it takes away the assumption of natural science that nature is the principle not only of motion but also of rest, which supposition makes it clear that rest is something natural, just as motion is. Hence, just as the first opinion, which destroyed motion, was against natural science, so too is this one that destroys rest.
The reason that he says that this opinion is less against the art is that rest is nothing more than the privation of motion. But that there is not a privation of motion more easily escapes notice than that there is no motion. For there are some motions so weak and insignificant that they can be scarcely noticed, and for that reason, it is easy to suppose that something is at rest when it really is not. But great and strong motions cannot be concealed; hence, it cannot be said that the senses are deceived in perceiving motion as they are in perceiving rest.
Second, therefore, at “moreover, the view is actually held” (253b9), he shows how some posited this second opinion. And he says that some, such as Heraclitus and his supporters, have said that all things that exist are always in motion, and not some things only, or just at some time, but that this motion eludes our senses. Now, if they say this of some motions, they are correct, for some motions do elude us. But, because they do not qualify their statement, but speak of all motions, it is not hard to find arguments against them, for there are many motions that evidently could not have existed always.
1008. Third, at “thus we may point out” (253b13), he forms the arguments against this position:
first, with respect to the motion of growth;
second, with respect to the motion of alteration, at “similarly, too, in the case” (253b23; [1009]);
third, with respect to local motion, at “again, in the matter of locomotion” (253b31; [1012]).
The reason he begins with growth is that Heraclitus was led to his doctrine as a result of considering growth. For he observed that a person grows a small amount in one year, and supposing that growth is continuous, he believed that, in each part of that year, he was increased with respect to part of that quantity, yet with that increase not sensed because it comes in a small portion of time. He reasoned, therefore, that the same thing happens in other things that seem to be at rest.
Against this, Aristotle says that it is not possible for a thing to be continually increased or diminished such that the increased quantity can be divided according to time in such a way that, in each part of time, there is a corresponding increase. Rather, there is, after the increase of one part, a time in which there is no increase, but a disposition is produced for the increase of the next part.
And this he explains with similar examples. The first of these is that we see that the multiplication of drops of rain wears away a stone. The second example is that we see that plants growing in stones divide the stones. Now, we cannot say that, if the repeated drops dig out or remove a certain quantity of the stone in a given time, half of this number of drops in half the time would previously remove half of that quantity. But what happens here is what happens with regard to ship haulers. For it does not follow that, if one hundred men pull a ship a certain distance in a given time, then fifty of them will move it half the distance in the same time or the full distance in twice the time—this was said in book 7. So also it does not follow that, if many drops hollow out a stone, then some part of those drops had previously removed the half in some certain time.
The reason for this is that what is removed from the stone by many drops is indeed divisible into many parts, but none of them is removed separately from the stone, for all the parts are removed at once, in the sense that they are in the totality removed in potency.
And he is speaking here of the first total quantity that is removed, for there is nothing to prevent that, when, over a long period of time, such a large quantity be removed from the stone by these drops, a certain part may have been removed previously by a part of these drops. But we must come to a removed quantity that is removed all together and not part after part. Therefore, in the removing of that whole, none of the preceding drops removed anything, but merely disposed for its removal. However, the last acts in virtue of all and removes what the others had disposed to be removed.
The same is true in the motion called decrease. For it is not necessary that, if something decreases a certain amount in a given time (even though the quantity be divided to infinity), then in every part of that time, a corresponding part of the removed quantity should depart; rather, at some time, a given amount will depart all together. The same holds in increase. Consequently, it is not required that something be continuously increased or decreased.
1009. Then, at “similarly, too, in the case” (253b23), he contradicts the above-mentioned position of continuous motion with respect to alteration, and this with three arguments.
First, he says that what was said about increase applies also to alteration. For although a body that is being altered is infinitely divisible, that is no reason for supposing that alteration is divided to infinity such that, for each period of time, a part of the alteration should occur. Rather, alteration very often takes place swiftly; that is, many parts of the altered body are altered all at once, as happens when water is condensed or frozen. For a whole mass of water is frozen all at once and not part after part (although, if it be a large mass of water, there is nothing to prevent part freezing after part).
It should be noticed that what Aristotle says here about alteration and growth seems contrary to what was said in book 6, where it was shown that motion is divided according to the division of the time, and of the mobile, and of that in respect of which there is motion.
But it should be recognized that, in book 6, Aristotle was talking about motion in common, without application to definite mobiles. Therefore, what he discussed there must be taken according to the requirements of motion’s continuity, but at present, he is speaking of motion with application to definite mobiles, in which a motion can be interrupted and not be continuous, which, when viewed under the common account, could be continuous.
1010. He gives the second argument at “again, when anyone has fallen ill” (253b26), and he says that, if a sick person is to get well, he has to be healed in a period of time, and not in an instant of the time. And it is further necessary that the very change, which is healing, tend to a definite terminus, namely, to health, and not to anything else. Accordingly, every alteration requires a definite time and a definite terminus, because every alteration tends to a contrary, as was said in book 5. But no such change is always continuous. Therefore, to say that something is being always and continuously altered is to speak against the facts.
1011. The third argument he gives at “moreover, we notice” (253b30), and he says that a stone does not become harder or softer, even after a very great period of time; thus, it is foolish to say that all things are always being altered.
1012. Then, at “again, in the matter of locomotion” (253b31), he contradicts the opinion at issue with respect to local motion, on two counts.
The first is that some local motions and rests are so evident that they cannot be hidden. For it would be strange if it were hidden from us when a stone is carried downwards or when it is at rest on the earth. Consequently, it cannot be said that, because of the concealment of local motions, all things should be supposed to be always being moved locally.
1013. Second, at “further, it is a law” (253b33), he argues thus: earth and any other natural body, when they are in their due places, rest from a necessity of nature and are not removed except by force. But it is evident that certain natural bodies are in their due place. Therefore, it is necessary to say that some things are at rest with respect to place and that not all things are being moved locally.
Finally, he concludes in summary that, from the foregoing and other things similar to the foregoing, anyone can know that it is impossible for all things always to be in motion as Heraclitus said, or for all things always to be at rest as Zeno and Parmenides and Melissus said.