on the heavens-第17章
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led to assert that the earth below us is infinite； saying； with Xenophanes of Colophon； that it has 'pushed its roots to infinity'；…in order to save the trouble of seeking for the cause。 Hence the sharp rebuke of Empedocles； in the words 'if the deeps of the earth are endless and endless the ample ether…such is the vain tale told by many a tongue； poured from the mouths of those who have seen but little of the whole。 Others say the earth rests upon water。 This； indeed； is the oldest theory that has been preserved； and is attributed to Thales of Miletus。 It was supposed to stay still because it floated like wood and other similar substances； which are so constituted as to rest upon but not upon air。 As if the same account had not to be given of the water which carries the earth as of the earth itself！ It is not the nature of water； any more than of earth； to stay in mid…air： it must have something to rest upon。 Again； as air is lighter than water； so is water than earth： how then can they think that the naturally lighter substance lies below the heavier？ Again； if the earth as a whole is capable of floating upon water； that must obviously be the case with any part of it。 But observation shows that this is not the case。 Any piece of earth goes to the bottom； the quicker the larger it is。 These thinkers seem to push their inquiries some way into the problem； but not so far as they might。 It is what we are all inclined to do； to direct our inquiry not by the matter itself； but by the views of our opponents： and even when interrogating oneself one pushes the inquiry only to the point at which one can no longer offer any opposition。 Hence a good inquirer will be one who is ready in bringing forward the objections proper to the genus； and that he will be when he has gained an understanding of all the differences。 Anaximenes and Anaxagoras and Democritus give the flatness of the earth as the cause of its staying still。 Thus； they say； it does not cut； but covers like a lid； the air beneath it。 This seems to be the way of flat…shaped bodies： for even the wind can scarcely move them because of their power of resistance。 The same immobility； they say； is produced by the flatness of the surface which the earth presents to the air which underlies it； while the air； not having room enough to change its place because it is underneath the earth； stays there in a mass； like the water in the case of the water…clock。 And they adduce an amount of evidence to prove that air； when cut off and at rest； can bear a considerable weight。 Now； first； if the shape of the earth is not flat； its flatness cannot be the cause of its immobility。 But in their own account it is rather the size of the earth than its flatness that causes it to remain at rest。 For the reason why the air is so closely confined that it cannot find a passage； and therefore stays where it is； is its great amount： and this amount great because the body which isolates it； the earth； is very large。 This result； then； will follow； even if the earth is spherical； so long as it retains its size。 So far as their arguments go； the earth will still be at rest。 In general； our quarrel with those who speak of movement in this way cannot be confined to the parts； it concerns the whole universe。 One must decide at the outset whether bodies have a natural movement or not； whether there is no natural but only constrained movement。 Seeing； however； that we have already decided this matter to the best of our ability； we are entitled to treat our results as representing fact。 Bodies； we say； which have no natural movement； have no constrained movement； and where there is no natural and no constrained movement there will be no movement at all。 This is a conclusion； the necessity of which we have already decided； and we have seen further that rest also will be inconceivable； since rest； like movement； is either natural or constrained。 But if there is any natural movement； constraint will not be the sole principle of motion or of rest。 If； then； it is by constraint that the earth now keeps its place； the so…called 'whirling' movement by which its parts came together at the centre was also constrained。 （The form of causation supposed they all borrow from observations of liquids and of air； in which the larger and heavier bodies always move to the centre of the whirl。 This is thought by all those who try to generate the heavens to explain why the earth came together at the centre。 They then seek a reason for its staying there； and some say； in the manner explained； that the reason is its size and flatness； others； with Empedocles； that the motion of the heavens； moving about it at a higher speed； prevents movement of the earth； as the water in a cup； when the cup is given a circular motion； though it is often underneath the bronze； is for this same reason prevented from moving with the downward movement which is natural to it。） But suppose both the 'whirl' and its flatness （the air beneath being withdrawn） cease to prevent the earth's motion； where will the earth move to then？ Its movement to the centre was constrained； and its rest at the centre is due to constraint； but there must be some motion which is natural to it。 Will this be upward motion or downward or what？ It must have some motion； and if upward and downward motion are alike to it； and the air above the earth does not prevent upward movement； then no more could air below it prevent downward movement。 For the same cause must necessarily have the same effect on the same thing。 Further； against Empedocles there is another point which might be made。 When the elements were separated off by Hate； what caused the earth to keep its place？ Surely the 'whirl' cannot have been then also the cause。 It is absurd too not to perceive that； while the whirling movement may have been responsible for the original coming together of the art of earth at the centre； the question remains； why now do all heavy bodies move to the earth。 For the whirl surely does not come near us。 Why； again； does fire move upward？ Not； surely； because of the whirl。 But if fire is naturally such as to move in a certain direction； clearly the same may be supposed to hold of earth。 Again； it cannot be the whirl which determines the heavy and the light。 Rather that movement caused the pre…existent heavy and light things to go to the middle and stay on the surface respectively。 Thus； before ever the whirl began； heavy and light existed； and what can have been the ground of their distinction； or the manner and direction of their natural movements？ In the infinite chaos there can have been neither above nor below； and it is by these that heavy and light are determined。 It is to these causes that most writers pay attention： but there are some； Anaximander； for instance； among the ancients； who say that the earth keeps its place because of its indifference。 Motion upward and downward and sideways were all； they thought； equally inappropriate to that which is set at the centre and indifferently related to every extreme point； and to move in contrary directions at the same time was impossible： so it must needs remain still。 This view is ingenious but not true。 The argument would prove that everything； whatever it be； which is put at the centre； must stay there。 Fire； then； will rest at the centre： for the proof turns on no peculiar property of earth。 But this does not follow。 The observed facts about earth are not only that it remains at the centre； but also that it moves to the centre。 The place to which any fragment of earth moves must necessarily be the place to which the whole moves； and in the place to which a thing naturally moves； it will naturally rest。 The reason then is not in the fact that the earth is indifferently related to every extreme point： for this would apply to any body； whereas movement to the centre is peculiar to
earth。 Again it is absurd to look for a reason why the earth remains at the centre and not for a reason why fire remains at the extremity。 If the extremity is the natural place of fire； clearly earth must also have a natural place。 But suppose that the centre is not its place； and that the reason of its remaining there is this necessity of indifference…on the
