on the heavens-第13章
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e； will be shorter than either of the radii。 Therefore the place in which it terminates will be a hollow place。 The water then will collect there until equality is established； that is until the line AE is equal to the two radii。 Thus water forces its way to the ends of the radii； and there only will it rest： but the line which connects the extremities of the radii is circular： therefore the surface of the water BEC is spherical。 It is plain from the foregoing that the universe is spherical。 It is plain； further； that it is turned （so to speak） with a finish which no manufactured thing nor anything else within the range of our observation can even approach。 For the matter of which these are composed does not admit of anything like the same regularity and finish as the substance of the enveloping body； since with each step away from earth the matter manifestly becomes finer in the same proportion as water is finer than earth。
5
Now there are two ways of moving along a circle； from A to B or from A to C； and we have already explained that these movements are not contrary to one another。 But nothing which concerns the eternal can be a matter of chance or spontaneity； and the heaven and its circular motion are eternal。 We must therefore ask why this motion takes one direction and not the other。 Either this is itself an ultimate fact or there is an ultimate fact behind it。 It may seem evidence of excessive folly or excessive zeal to try to provide an explanation of some things； or of everything； admitting no exception。 The criticism； however； is not always just： one should first consider what reason there is for speaking； and also what kind of certainty is looked for； whether human merely or of a more cogent kind。 When any one shall succeed in finding proofs of greater precision； gratitude will be due to him for the discovery； but at present we must be content with a probable solution。 If nature always follows the best course possible； and； just as upward movement is the superior form of rectilinear movement； since the upper region is more divine than the lower； so forward movement is superior to backward； then front and back exhibits； like right and left； as we said before and as the difficulty just stated itself suggests； the distinction of prior and posterior； which provides a reason and so solves our difficulty。 Supposing that nature is ordered in the best way possible； this may stand as the reason of the fact mentioned。 For it is best to move with a movement simple and unceasing； and； further； in the superior of two possible directions。
6
We have next to show that the movement of the heaven is regular and not irregular。 This applies only to the first heaven and the first movement； for the lower spheres exhibit a composition of several movements into one。 If the movement is uneven； clearly there will be acceleration； maximum speed； and retardation； since these appear in all irregular motions。 The maximum may occur either at the starting…point or at the goal or between the two； and we expect natural motion to reach its maximum at the goal； unnatural motion at the starting…point； and missiles midway between the two。 But circular movement； having no beginning or limit or middle in the direct sense of the words； has neither whence nor whither nor middle： for in time it is eternal； and in length it returns upon itself without a break。 If then its movement has no maximum； it can have no irregularity； since irregularity is produced by retardation and acceleration。 Further； since everything that is moved is moved by something； the cause of the irregularity of movement must lie either in the mover or in the moved or both。 For if the mover moved not always with the same force； or if the moved were altered and did not remain the same； or if both were to change； the result might well be an irregular movement in the moved。 But none of these possibilities can be conceived as actual in the case of the heavens。 As to that which is moved； we have shown that it is primary and simple and ungenerated and indestructible and generally unchanging； and the mover has an even better right to these attributes。 It is the primary that moves the primary； the simple the simple； the indestructible and ungenerated that which is indestructible and ungenerated。 Since then that which is moved； being a body； is nevertheless unchanging； how should the mover； which is incorporeal； be changed？ It follows then； further； that the motion cannot be irregular。 For if irregularity occurs； there must be change either in the movement as a whole； from fast to slow and slow to fast； or in its parts。 That there is no irregularity in the parts is obvious； since； if there were； some divergence of the stars would have taken place before now in the infinity of time； as one moved slower and another faster： but no alteration of their intervals is ever observed。 Nor again is a change in the movement as a whole admissible。 Retardation is always due to incapacity； and incapacity is unnatural。 The incapacities of animals； age； decay； and the like； are all unnatural； due； it seems； to the fact that the whole animal complex is made up of materials which differ in respect of their proper places； and no single part occupies its own place。 If therefore that which is primary contains nothing unnatural； being simple and unmixed and in its proper place and having no contrary； then it has no place for incapacity； nor； consequently； for retardation or （since acceleration involves retardation） for acceleration。 Again； it is inconceivable that the mover should first show incapacity for an infinite time； and capacity afterwards for another infinity。 For clearly nothing which； like incapacity； unnatural ever continues for an infinity of time； nor does the unnatural endure as long as the natural； or any form of incapacity as long as the capacity。 But if the movement is retarded it must necessarily be retarded for an infinite time。 Equally impossible is perpetual acceleration or perpetual retardation。 For such movement would be infinite and indefinite； but every movement； in our view； proceeds from one point to another and is definite in character。 Again； suppose one assumes a minimum time in less than which the heaven could not complete its movement。 For； as a given walk or a given exercise on the harp cannot take any and every time； but every performance has its definite minimum time which is unsurpassable； so； one might suppose； the movement of the heaven could not be completed in any and every time。 But in that case perpetual acceleration is impossible （and； equally； perpetual retardation： for the argument holds of both and each）； if we may take acceleration to proceed by identical or increasing additions of speed and for an infinite time。 The remaining alternative is to say that the movement exhibits an alternation of slower and faster： but this is a mere fiction and quite inconceivable。 Further； irregularity of this kind would be particularly unlikely to pass unobserved； since contrast makes observation easy。 That there is one heaven； then； only； and that it is ungenerated and eternal； and further that its movement is regular； has now been sufficiently explained。
7
We have next to speak of the stars； as they are called； of their composition； shape； and movements。 It would be most natural and consequent upon what has been said that each of the stars should be composed of that substance in which their path lies； since； as we said； there is an element whose natural movement is circular。 In so saying we are only following the same line of thought as those who say that the stars are fiery because they believe the upper body to be fire； the presumption being that a thing is composed of the same stuff as that in which it is situated。 The warmth and light which proceed from them are caused by the friction set up in the air by their motion。 Movement tends to create fire in wood； stone； and iron； and with even more reason should it have that effect on air； a substance which is closer to fire than these。 An example is that of missiles； which as they move are themselves fired so strongly th
