not itself spatial in the sense of being extended. Monad and matter were thus represented as facts of the same kind or genus, having their difference only in their relative isolation or aggregation. But Leibniz repudiated this idea, and that not only by the spirit of his teaching, but in express words. Monads "are not ingredients or constituents of matter," he says, "but only _conditions_ of it." "Monads can no more be said to be parts of bodies, or to come in contact with them, or to compose them, than can souls or mathematical points." "Monads _per se_ have _no_ situation relative to one another." An increase in the number of created monads, he says again, if such a thing could be supposed, would no more increase the amount of matter in existence, than mathematical points added to a line would increase its length. And again: "There is no nearness or remoteness among monads; to say that they are gathered in a point or are scattered in space, is to employ mental fictions, _in trying to imagine what can only be thought_." The italicized words give the clew to the whole discussion. To make monads of the same order as corporeal phenomena, is to make them sensible, or capable of being imaged, or conditioned by space and time,--three phrases which are strictly correlative. But the monads can only be thought,--that is, their qualities are ideal, not sensible; they can be realized only by reason, not projected in forms having spatial outline and temporal habitation, that is, in images. Monads and material things, in other words, are facts of two distinct orders; they are related as the rational or spiritual and the physical or sensible. Matter is no more composed of monads than it is of thoughts or of logical principles. As Leibniz says over and over again: Matter, space, time, motion are only phenomena, although phenomena _bene fundata_,--phenomena, that is, having their rational basis and condition. The monads, on the other hand, are not appearances, they are realities. Having freed our minds from the supposition that it is in any way possible to form an image or picture of the monad; having realized that it is wholly false to suppose that monads occupy position in space, and then by their continuity fill it, and make extended matter,--we must attempt to frame a correct theory of the nature of matter and its relation to the monad. We shall do this only as we realize that "matter," so far as it has any reality, or so far as it has any real _fundamentum_, must be something ideal, or, in Leibniz's language, "metaphysical." As he says over and over again, the only realities are the substances or spiritual units of activity, to which the name "monad" is given. In the inquiry, then, after such reality as matter may have, we must betake ourselves to this unit of living energy.