GENERAL PRINCIPLES 41 



itself a representation of the whole universe from one 

 particular point of view, which differs to an infinitely 

 small degree from the representations contained in some 

 other Monad. If, then, any change, however slight, 

 takes place in the perception or representation of one 

 Monad, the continuity of the series will be broken and 

 we shall have two indiscernible Monads. But it is of 

 the very essence of the Monads to be ' living mirrors/ 

 'forces' (as distinct from masses), centres of appetition, 

 spontaneously unfolding a sequence of perceptions. Ac- 

 cordingly this change within the Monad does take place : 

 it is essential to its nature. The continuous order or 

 system of the Monads must therefore be destroyed, unless 

 we can say that any change within one Monad produces, 

 or is invariably accompanied by, correlative changes in 

 other Monads, of such a kind that the equilibrium of the 

 whole system is maintained. In other words, there must 

 be something of the nature of mutual influence, action 

 and reaction, between the various elements in the system. 

 If the system were a plenum of mass, this interaction 

 would be intelligible without further explanation. But, 

 as the Monads form a qualitative continuum of such a 

 kind that no part can really act upon another, a further 

 hypothesis is required to complete the theory. 



This hypothesis is Leibniz's system of the pre-estab- 

 lished harmony between substances. Though no true 

 substance can really act upon another, everything in the 

 universe takes place as if this mutual interaction were 

 real. Substances form a system, not of physical relations, 

 but of harmony or mutual compatibility. In the creation 

 of the world, the inner development of each Monad has 

 been so prearranged that all its changes are accompanied 

 by corresponding changes in others. The succession of 



Monads would for him have meant to fall into Spinoza's pantheism, 

 while, on the other hand, to give up the continuity of their series 

 would have meant having recourse to Atomism. And these he 

 regarded as equally irrational alternatives. Cf. this Introduction, 

 Part iv. p. 1 88. 



