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  then we can no longer say that it is nature's work: for if anything is

  of a certain character naturally, it either is so invariably and is

  not sometimes of this and sometimes of another character (e.g. fire,

  which travels upwards naturally, does not sometimes do so and

  sometimes not) or there is a ratio in the variation. It would be

  better, therefore, to say with Empedocles and any one else who may

  have maintained such a theory as his that the universe is

  alternately at rest and in motion: for in a system of this kind we

  have at once a certain order. But even here the holder of the theory

  ought not only to assert the fact: he ought to explain the cause of

  it: i.e. he should not make any mere assumption or lay down any

  gratuitous axiom, but should employ either inductive or

  demonstrative reasoning. The Love and Strife postulated by

  Empedocles are not in themselves causes of the fact in question, nor

  is it of the essence of either that it should be so, the essential

  function of the former being to unite, of the latter to separate. If

  he is to go on to explain this alternate predominance, he should

  adduce cases where such a state of things exists, as he points to

  the fact that among mankind we have something that unites men,

  namely Love, while on the other hand enemies avoid one another: thus

  from the observed fact that this occurs in certain cases comes the

  assumption that it occurs also in the universe. Then, again, some

  argument is needed to explain why the predominance of each of the

  two forces lasts for an equal period of time. But it is a wrong

  assumption to suppose universally that we have an adequate first

  principle in virtue of the fact that something always is so or

  always happens so. Thus Democritus reduces the causes that explain

  nature to the fact that things happened in the past in the same way as

  they happen now: but he does not think fit to seek for a first

  principle to explain this 'always': so, while his theory is right in

  so far as it is applied to certain individual cases, he is wrong in

  making it of universal application. Thus, a triangle always has its

  angles equal to two right angles, but there is nevertheless an

  ulterior cause of the eternity of this truth, whereas first principles

  are eternal and have no ulterior cause. Let this conclude what we have

  to say in support of our contention that there never was a time when

  there was not motion, and never will be a time when there will not

  be motion.

  2

  The arguments that may be advanced against this position are not

  difficult to dispose of. The chief considerations that might be

  thought to indicate that motion may exist though at one time it had

  not existed at all are the following:

  First, it may be said that no process of change is eternal: for

  the nature of all change is such that it proceeds from something to

  something, so that every process of change must be bounded by the

  contraries that mark its course, and no motion can go on to infinity.

  Secondly, we see that a thing that neither is in motion nor contains

  any motion within itself can be set in motion; e.g. inanimate things

  that are (whether the whole or some part is in question) not in motion

  but at rest, are at some moment set in motion: whereas, if motion

  cannot have a becoming before which it had no being, these things

  ought to be either always or never in motion.

  Thirdly, the fact is evident above all in the case of animate

  beings: for it sometimes happens that there is no motion in us and

  we are quite still, and that nevertheless we are then at some moment

  set in motion, that is to say it sometimes happens that we produce a

  beginning of motion in ourselves spontaneously without anything having

  set us in motion from without. We see nothing like this in the case of

  inanimate things, which are always set in motion by something else

  from without: the animal, on the other hand, we say, moves itself:

  therefore, if an animal is ever in a state of absolute rest, we have a

  motionless thing in which motion can be produced from the thing

  itself, and not from without. Now if this can occur in an animal,

  why should not the same be true also of the universe as a whole? If it

  can occur in a small world it could also occur in a great one: and

  if it can occur in the world, it could also occur in the infinite;

  that is, if the infinite could as a whole possibly be in motion or

  at rest.

  Of these objections, then, the first-mentioned motion to opposites

  is not always the same and numerically one a correct statement; in

  fact, this may be said to be a necessary conclusion, provided that

  it is possible for the motion of that which is one and the same to

  be not always one and the same. (I mean that e.g. we may question

  whether the note given by a single string is one and the same, or is

  different each time the string is struck, although the string is in

  the same condition and is moved in the same way.) But still, however

  this may be, there is nothing to prevent there being a motion that

  is the same in virtue of being continuous and eternal: we shall have

  something to say later that will make this point clearer.

  As regards the second objection, no absurdity is involved in the

  fact that something not in motion may be set in motion, that which

  caused the motion from without being at one time present, and at

  another absent. Nevertheless, how this can be so remains matter for

  inquiry; how it 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 the rest 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 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

  case-or rather we may perhaps say that it must necessarily 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 when animals are asleep: 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 at a later

  stage in our discussion.

  3

  Our enquiry will resolve itself at the outset into a consideration

  of the above-mentioned pro
blem-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 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 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

  and all received 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

  fundamental assumption with him that motion is ultimately referable to

  nature herself.

  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 in our course on physics it was laid down 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 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:

  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: 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, 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, 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. therefore, that alteration is

  continuous is 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: from the fact then 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.

  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 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 obvious, for on this theory there can be no such thing

  as increase: nor can there be any such thing as compulsory motion,

  if it is impossible that a thing can be at rest before being set in

  motion unnaturally. This 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 that to which a thing changes

  comes to be, or occupancy of it comes to be, and that from which a

  thing changes ceases to be, or there ceases to be occupancy of it.

  It is clear, therefore, that there are cases of occasional motion

  and occasional rest.

  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 as 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 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 repeat that

  assertion. We may point out that, even if it is really the case, as

  certain persons assert, that the existent 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 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 a

 
reasoned justification of a belief with regard to which we are too

  well off to require reasoned justification-implies bad judgement of

  what is better and what is worse, what commends itself to belief and

  what does not, what is ultimate 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 we see some things that are 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 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, to some the motion

  is accidental, to others essential: thus it is accidental to what

  merely belongs to or contains as a part a thing that causes motion

  or suffers motion, essential to a thing that causes motion or

  suffers motion not merely by belonging to such a thing or containing

  it as a part.

  Of things to which the motion is essential 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 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