Leibniz’s system had great merits and great demerits. The theory that “matter” is a confused way of perceiving something non-material was an advance upon anything to be found in his predecessors. He had, though only semi-consciously, the distinction between physical and perceptual space: there is space in each monad’s picture of the world, and there is also the assemblage or pattern of “points of view”. The latter corresponds to what I have called “physical space”, the former to “perceptual space”. Leibniz maintained, as against Newton, that space and time consists only of relations--a view which has achieved a definitive triumph in Einstein’s theory of relativity. The weak point of his system was what he called the “pre-established harmony”, in virtue of which all the monads (so to speak) kept step, in spite of the fact that they were “windowless” and never acted upon each other. Perception, for Leibniz, was not an effect of the object perceived, but a modification arising in the perceiving monad and running parallel with what was happening in the perceived object. This view would never have seemed plausible but for the anterior Cartesian theory of the mutual independence of mind and matter. And if Leibniz himself developed, as he believed, in complete independence of all other created things, it is not clear what good reasons he could have had for believing in the existence of anything except himself, since, by his own theory, his experiences would remain unchanged if everything else were annihilated. In fact, he was only able to refute this possibility by bringing in theological considerations, which, whether valid or not, are out of place in philosophy. For this reason, his doctrines, ingenious as they were, found little acceptance in France and England, though in Germany they prevailed, in a modified form, until the time of Kant.