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Leibniz has, however, a complete philosophy of nature. In his other writing, he explains the ideas of matter and force in their dependence upon his metaphysic, or doctrine of spiritual entelechies. The task does not at first sight appear an easy one. The reality, according to Leibniz, is purely spiritual, does not exist in space nor time, and is a principle of activity following its own law,—that of reflecting the universe of spiritual relations. How from this world of ideal, unextended, and non-temporal dynamic realities we are to pass over to a material world of extension, with its static existence in space, and transitory passage in time, is a question challenging the whole Leibnizian system. It is a question, however, for which Leibniz himself has provided an answer. We may not regard it as adequate; we may think that he has not truly derived the material world from his spiritual principles: but at all events he asked himself the question, and gave an answer. We shall investigate this answer by arranging what Leibniz has said under the heads of: matter as a metaphysical principle; matter as a physical phenomenon; and the relation of phenomena to absolute reality, or of the physical to the metaphysical. In connection with the second head, particularly, we shall find it necessary to discuss what Leibniz has said about space, time, and motion.

Wolff, who put the ideas of Leibniz into systematic shape, did it at the expense of almost all their significance. He took away the air of paradox, of remoteness, that characterized Leibniz's thought, and gave it a popular form. But its depth and suggestiveness vanished in the process. Unfortunately, Wolff's presentations of the philosophy of Leibniz have been followed by others, to whom it seemed a dull task to follow out the intricacies of a thought nowhere systematically expressed. This has been especially the case as concerns the Leibnizian doctrine of matter. A superficial interpretation of certain passages in Leibniz has led to an almost universal misunderstanding about it. Leibniz frequently says that since matter is composite or complex, it follows that there must be something simple as its basis, and this simple something is the monad. The misinterpretation just spoken of consists in supposing that Leibniz meant that matter as composite is made up of monads as simple; that the monad and matter are facts of the same order, the latter being only an aggregate, or continued collection of the former. It interpreted the conception of Leibniz in strict analogy with the atomic theory of Lucretius, excepting that it granted that the former taught that the ultimate atom, the component of all complex forms of matter, has position only, not extension, its essence consisting in its exercise of force, not in its mere space occupancy. The monad was thus considered to be in space, or at least conditioned by space relations, as is a mathematical point, although 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.

Although every monad is active, it is not entirely active. There is, as we have already seen, an infinite scale of substances; and since substance is equivalent to activity, this is saying that there is an infinite scale of activities. God alone is purus actus, absolute energy, untouched by passivity or receptivity. Every other being has the element of incompleteness, of inadequacy; it does not completely represent the universe. In this passivity consists its finitude, so that Leibniz says that not even God himself could deprive monads of it, for this would be to make them equal to himself. In this passivity, incompleteness, or finitude, consists what we call matter. Leibniz says that he can understand what Plato meant when he called matter something essentially imperfect and transitory. Every finite monad is a union of two principles,—those of activity and of passivity. "I do not admit," says Leibniz, "that there are souls existing simply by themselves, or that there are created spirits detached from all body. God alone is above all matter, since he is its author; creatures freed from matter would be at the same time detached from the universal connection of things, and, as it were, deserters from the general order." And again, "Beings have a nature which is both active and passive; that is, material and immaterial." And again, he says that every created monad requires both an entelechy, or principle of activity, and matter. "Matter is essential to any entelechy, and can never be separated from it, since matter completes it." In short, the term "monad" is equivalent to the term "entelechy" only when applied to God. In every other monad, the entelechy, or energy, is but one factor. "Matter, or primitive passive power, completes the entelechy, or primitive active power, so that it becomes a perfect substance, or monad." On the other hand, of course, matter, as the passive principle, is a mere potentiality or abstraction, considered in itself. It is real only in its union with the active principle. Matter, he says, "cannot exist without immaterial substances." "To every particular portion of matter belongs a particular form; that is, a soul, a spirit." To this element of matter, considered as an abstraction, in its distinction from soul, Leibniz, following the scholastics, and ultimately Aristotle, gives the name, "first" or "bare" matter. The same influence is seen in the fact that he opposes this element of matter to "form," or the active principle.