go INTRODUCTION 



in the world is absolutely at rest. Accordingly no body 

 begins to move from a state of absolute rest, but from 

 a state which is to be conceived as already one of motion, 

 however small in amount. Actual motion is not some- 

 thing added to a body which, to begin with, is bare mass ; 

 it is always gradual growth or increment of a motion 

 which is already there. Actual motion always pre-sup- 

 poses potential motion or a force which, though it may 

 not be observed, tends to appear as actual motion. 

 Descartes, then, was right in interpreting actual motion 

 as change of position, but wrong in overlooking potential 

 motion and thus in regarding the total quantity of actual 

 apparent motion in the universe, or in any independent 

 system, as constant. He was right also in holding that 

 each body tends to continue in the state in which it is ; 

 but he was wrong in thinking that a body can ever be in 

 a state of absolute rest, and thus in supposing that one 

 motion cannot oppose another, but can only be opposed 

 by rest. As a matter of fact everything tends to move, 

 and would move, were it not for counteracting tendencies 

 to motion in other things 1 . That which is conserved, then, 

 is not actual motion, as an extrinsic property of material 

 substance, but this intrinsic tendency or potentiality of 

 motion, which Leibniz calls force. As mere change of 

 position does not enable us to attribute motion to one of 

 the two bodies whose position changes, and not to the 

 other, the body which we call the moving body (as dis- 

 tinct from the body at rest) is so, not in virtue of its 

 motion (in the sense of change of position), but because it 



1 Cf. Lettre a M. Pelisson (1691) (Foucher de Careil, i. 208 ; Dutens, 

 i- 733) : ' It must be observed that every body makes an effort to 

 act on outside things, and would perceptibly act if the contrary 

 efforts of surrounding bodies did not prevent it. This has not been 

 sufficiently noticed by our moderns. They imagine that a body 

 might be perfectly at rest, without any effort. But this is due to 

 their failure to understand what bodily substance really is ; for in 

 my opinion substance cannot (at any rate naturally) be without action. 

 This also disproves the inaction which Socinians attribute to dis- 

 embodied souls.' 



