STATEMENT OF LEIBNIZ S PHILOSOPHY 9! 



contains within itself the cause of the change, the force or 

 activity which produces the motion. ' The notion of 

 force,' says Leibniz, 'is as clear as that of activity and of 

 passivity, for it is that from which activity follows, when 

 nothing prevents it. It is eifort, conatus ; and while 

 motion is a successive thing, which consequently never 

 exists, any more than time, because all its parts never 

 exist together while, I say, that is so, force or eifort, on 

 the other hand, exists quite completely at every instant 

 and must be something genuine and real. And, as nature 

 has to do rather with the real than with that which does 

 not completely exist except in our mind, it appears (in 

 consequence of what I have shown) that it is the same 

 quantity of force, and not (as Descartes believed) the same 

 quantity of motion, that is preserved in nature V 



This force, then, which is constant, is not only an 

 actual but a potential reality. It is not mere capacity 

 for motion, mere passive movableness, nor is it actual 

 manifest motion or activity in general. It is something 

 between the two, an undeveloped or restrained tendency 

 to act, which in appropriate circumstances is the producer 

 of action 2 . This force is to be measured by the quantity 

 of effect it produces. Descartes rightly insisted on the 

 quantity of effect as the thing to be measured ; but he 



1 Lettre a M. Pelisson (no date, probably 1691) (Dutens, i. 719 ; 

 Foucher de Careil, i. 157). ' The relative velocity of two bodies ' 

 [i. e. their apparent motion] ' may remain the same, although the 

 real velocities and absolute forces of the bodies change in an infinity 

 of ways, so that conservation of relative velocity has nothing to do 

 with what is absolute in the bodies.' Essai de dynamique (Gr. Math, 

 vi. 216). Cf. Appendix I, p. 351. 



2 Cf. De Primae Philosophiae Emendatione, &c. (1694) (E. 122 b ; G. iv 

 469) : l Active force differs from the bare potency commonly recog- 

 nized in the Schools. For the active potency of the Scholastics, or 

 faculty, is nothing but a mere possibility of acting, which neverthe- 

 less requires an outer excitation or stimulus, that it may be turned 

 into activity. But active force contains a certain activity [actus] 

 and is a mean between the faculty of acting and action itself. It 

 includes effort and thus passes into operation by itself, requiring no 

 aids, but only the removal of hindrance. This may be illustrated 

 by the example of a heavy hanging body stretching the rope which 

 holds it up, or by that of a drawn bow.' 



