MOVEMENT OF ANIMALS, iii.-iv. 



motion in that which is first at rest, so that the force 

 exerted is greater than the immobility rather than 

 similar and equal to it, and likewise greater than the 

 force of that which is moved but does not create 

 movement. Therefore the power of the earth in its 

 immobility will necessarily be as great as that pos- 

 sessed by the whole heaven and that which sets it in 

 motion. If, however, this is impossible, the move- 

 ment of the heaven by any such force within it is also 

 impossible. 



IV. A problem also arises about the movements 

 of the parts of the heaven, which might well be dis- 

 cussed, since it is closely connected with what has 

 been said above. If one were to overmaster the im- 

 mobihty of the earth by the power of motion, one will 

 obviously move it away from the centre. ° Moreover 

 it is clear that the force from which this power is de- 

 rived is not infinite ; for the earth is not infinite, and 

 so its weight is not infinite either. Now^ the w^ord 

 " impossible " is used in several senses (we are using 

 it in different senses when we say that it is impossible 

 to see a sound, and when we say that it is impossible 

 for us to see the men in the moon ; for the former is 

 of necessity invisible, the latter are of such a nature 

 as to be seen but will never be seen by us), but w^e 

 hold that the heaven is of necessity impossible to 

 destroy and dissolve, whereas according to our present 

 argument it is not necessarily so ; for it is within the 

 nature of things and the bounds of possibility that a 

 motive force should exist greater both than that 

 which causes the earth to be at rest and than that 

 which causes the fire and upper body ^ to move. 

 If, therefore, the overpowering motive forces exist, 

 these will be dissolved by one another; but if they 



p2 451 



