No two monads, he held, can ever have any causal relation to each other; when it seems as if they had, appearances are deceptive. Monads, as he expressed it, are ‘windowless’. This led to two difficulties: one in dynamics, where bodies seem to affect each other, especially in impact; the other in relation to perception, which seems to be an effect of the perceived object upon the percipient. We will ignore the dynamical difficulty for the present, and consider only the question of perception. Leibniz held that every monad mirrors the universe, not because the universe affects it, but because God has given it a nature which spontaneously produces this result. There is a ‘pre-established harmony’ between the changes in one monad and those in another, which produces the semblance of interaction.

Bertrand Russell, History of Western Philosop…, loc. 2129