on the heavens-第15节
按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
for the observed facts on their view that we do not dwell at the centre； than on the common view that the earth is in the middle。 Even as it is； there is nothing in the observations to suggest that we are removed from the centre by half the diameter of the earth。 Others； again； say that the earth； which lies at the centre； is 'rolled'； and thus in motion； about the axis of the whole heaven； So it stands written in the Timaeus。 III。 There are similar disputes about the shape of the earth。 Some think it is spherical； others that it is flat and drum…shaped。 For evidence they bring the fact that； as the sun rises and sets； the part concealed by the earth shows a straight and not a curved edge； whereas if the earth were spherical the line of section would have to be circular。 In this they leave out of account the great distance of the sun from the earth and the great size of the circumference； which； seen from a distance on these apparently small circles appears straight。 Such an appearance ought not to make them doubt the circular shape of the earth。 But they have another argument。 They say that because it is at rest； the earth must necessarily have this shape。 For there are many different ways in which the movement or rest of the earth has been conceived。 The difficulty must have occurred to every one。 It would indeed be a complacent mind that felt no surprise that； while a little bit of earth； let loose in mid…air moves and will not stay still； and more there is of it the faster it moves； the whole earth； free in midair； should show no movement at all。 Yet here is this great weight of earth； and it is at rest。 And again； from beneath one of these moving fragments of earth； before it falls； take away the earth； and it will continue its downward movement with nothing to stop it。 The difficulty then； has naturally passed into a common place of philosophy； and one may well wonder that the solutions offered are not seen to involve greater absurdities than the problem itself。 By these considerations some have been 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 e
