on the heavens-第23章
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ommon error of all views which assume a single element is that they allow only one natural movement； which is the same for every body。 For it is a matter of observation that a natural body possesses a principle of movement。 If then all bodies are one； all will have one movement。 With this motion the greater their quantity the more they will move； just as fire； in proportion as its quantity is greater； moves faster with the upward motion which belongs to it。 But the fact is that increase of quantity makes many things move the faster downward。 For these reasons； then； as well as from the distinction already established of a plurality of natural movements； it is impossible that there should be only one element。 But if the elements are not an infinity and not reducible to one； they must be several and finite in number。
6
First we must inquire whether the elements are eternal or subject to generation and destruction； for when this question has been answered their number and character will be manifest。 In the first place； they cannot be eternal。 It is a matter of observation that fire； water； and every simple body undergo a process of analysis； which must either continue infinitely or stop somewhere。 （1） Suppose it infinite。 Then the time occupied by the process will be infinite； and also that occupied by the reverse process of synthesis。 For the processes of analysis and synthesis succeed one another in the various parts。 It will follow that there are two infinite times which are mutually exclusive； the time occupied by the synthesis； which is infinite； being preceded by the period of analysis。 There are thus two mutually exclusive infinites； which is impossible。 （2） Suppose； on the other hand； that the analysis stops somewhere。 Then the body at which it stops will be either atomic or； as Empedocles seems to have intended； a divisible body which will yet never be divided。 The foregoing arguments show that it cannot be an atom； but neither can it be a divisible body which analysis will never reach。 For a smaller body is more easily destroyed than a larger； and a destructive process which succeeds in destroying； that is； in resolving into smaller bodies； a body of some size； cannot reasonably be expected to fail with the smaller body。 Now in fire we observe a destruction of two kinds： it is destroyed by its contrary when it is quenched； and by itself when it dies out。 But the effect is produced by a greater quantity upon a lesser； and the more quickly the smaller it is。 The elements of bodies must therefore be subject to destruction and generation。 Since they are generated； they must be generated either from something incorporeal or from a body； and if from a body； either from one another or from something else。 The theory which generates them from something incorporeal requires an extra…corporeal void。 For everything that comes to be comes to be in something； and that in which the generation takes place must either be incorporeal or possess body； and if it has body； there will be two bodies in the same place at the same time； viz。 that which is coming to be and that which was previously there； while if it is incorporeal； there must be an extra…corporeal void。 But we have already shown that this is impossible。 But； on the other hand； it is equally impossible that the elements should be generated from some kind of body。 That would involve a body distinct from the elements and prior to them。 But if this body possesses weight or lightness； it will be one of the elements； and if it has no tendency to movement； it will be an immovable or mathematical entity； and therefore not in a place at all。 A place in which a thing is at rest is a place in which it might move； either by constraint； i。e。 unnaturally； or in the absence of constraint； i。e。 naturally。 If； then； it is in a place and somewhere； it will be one of the elements； and if it is not in a place； nothing can come from it； since that which comes into being and that out of which it comes must needs be together。 The elements therefore cannot be generated from something incorporeal nor from a body which is not an element； and the only remaining alternative is that they are generated from one another。
7
We must； therefore； turn to the question； what is the manner of their generation from one another？ Is it as Empedocles and Democritus say； or as those who resolve bodies into planes say； or is there yet another possibility？ （1） What the followers of Empedocles do； though without observing it themselves； is to reduce the generation of elements out of one another to an illusion。 They make it a process of excretion from a body of what was in it all the time…as though generation required a vessel rather than a material…so that it involves no change of anything。 And even if this were accepted； there are other implications equally unsatisfactory。 We do not expect a mass of matter to be made heavier by compression。 But they will be bound to maintain this； if they say that water is a body present in air and excreted from air； since air becomes heavier when it turns into water。 Again； when the mixed body is divided； they can show no reason why one of the constituents must by itself take up more room than the body did： but when water turns into air； the room occupied is increased。 The fact is that the finer body takes up more room； as is obvious in any case of transformation。 As the liquid is converted into vapour or air the vessel which contains it is often burst because it does not contain room enough。 Now； if there is no void at all； and if； as those who take this view say； there is no expansion of bodies； the impossibility of this is manifest： and if there is void and expansion； there is no accounting for the fact that the body which results from division cfpies of necessity a greater space。 It is inevitable； too； that generation of one out of another should come to a stop； since a finite quantum cannot contain an infinity of finite quanta。 When earth produces water something is taken away from the earth； for the process is one of excretion。 The same thing happens again when the residue produces water。 But this can only go on for ever； if the finite body contains an infinity； which is impossible。 Therefore the generation of elements out of one another will not always continue。 （2） We have now explained that the mutual transformations of the elements cannot take place by means of excretion。 The remaining alternative is that they should be generated by changing into one another。 And this in one of two ways； either by change of shape； as the same wax takes the shape both of a sphere and of a cube； or； as some assert； by resolution into planes。 （a） Generation by change of shape would necessarily involve the assertion of atomic bodies。 For if the particles were divisible there would be a part of fire which was not fire and a part of earth which was not earth； for the reason that not every part of a pyramid is a pyramid nor of a cube a cube。 But if （b） the process is resolution into planes； the first difficulty is that the elements cannot all be generated out of one another。 This they are obliged to assert； and do assert。 It is absurd； because it is unreasonable that one element alone should have no part in the transformations； and also contrary to the observed data of sense； according to which all alike change into one another。 In fact their explanation of the observations is not consistent with the observations。 And the reason is that their ultimate principles are wrongly assumed： they had certain predetermined views； and were resolved to bring everything into line with them。 It seems that perceptible things require perceptible principles； eternal things eternal principles； corruptible things corruptible principles； and； in general； every subject matter principles homogeneous with itself。 But they； owing to their love for their principles； fall into the attitude of men who undertake the defence of a position in argument。 In the confidence that the principles are true they are ready to accept any consequence of their application。 As though some principles did not require to be judged from their results； and particularly fr
