Leibniz finds the best or completest demonstration of the spiritual nature of the ultimate unity. This is found in the use which can be made of the hypothesis. The truest witness to the spiritual character of reality is found in the capacity of this principle to comprehend and explain the facts of experience. With this conception the reason of things can be ascertained, and light introduced into what were otherwise a confused obscurity. And, indeed, this is the only sufficient proof of any doctrine. It is not what comes before the formulation of a theory which proves it; it is not the facts which suggest it, or the processes which lead up to it: it is what comes after the formation of the theory,—the uses that it can be put to; the facts which it will render significant. The whole philosophy of Leibniz in its simplicity, width, and depth, is the real evidence of the truth of his philosophical principle.
The monad, then, is a spiritual unity; it is individualized life. Unity, activity, individuality are synonymous terms in the vocabulary of Leibniz. Every unity is a true substance, containing within itself the source and law of its own activity. It is that which is internally determined to action. It is to be conceived after the analogy of the soul. It is an indivisible unity, like “that particular something in us which thinks, apperceives and wills, and distinguishes us in a way of its own from whatever else thinks and wills.” Against Descartes, therefore, Leibniz stands for the principle of unity; against Spinoza, he upholds the doctrine of individuality, of diversity, of multiplicity. And the latter principle is as important in his thought as the former. Indeed, they are inseparable. The individual is the true unity. There is an infinite number of these individuals, each distinct from every other. The law of specification, of distinction, runs through the universe. Two beings cannot be alike. They are not individualized merely by their different positions in space or time; duration and extension, on the contrary, are, as we have seen, principles of relativity, of connection. Monads are specified by an internal principle. Their distinct individuality is constituted by their distinct law of activity. Leibniz will not have a philosophy of abstract unity, representing the universe as simple only, he will have a philosophy equal to the diversity, the manifold wealth of variety, in the universe. This is only to say that he will be faithful to his fundamental notion,—that of Life. Life does not mean a simple unity like a mathematical one, it means a unity which is the harmony of the interplay of diverse organs, each following its own law and having its own function. When Leibniz says, God willed to have more monads rather than fewer, the expression is indeed one of naïveté, but the thought is one of unexplored depth. It is the thought that Leibniz repeats when he says, “Those who would reduce all things to modifications of one universal substance do not have sufficient regard to the order, the harmony of reality.” Leibniz applies here, as everywhere, the principle of continuity, which is unity in and through diversity, not the principle of bare oneness. There is a kingdom of monads, a realm truly infinite, composed of individual unities or activities in an absolute continuity. Leibniz was one of the first, if not the first, to use just the expression “uniformity of nature;” but even here he explains that it means “uniform in variety, one in principle, but varied in manifestation.” The world is to be as rich as possible. This is simply to say that distinct individuality as well as ultimate unity is a law of reality.
But has not Leibniz fallen into a perilous position? In avoiding the monotone of unity which characterizes the thought of Spinoza, has he not fallen into a lawless variety of multiplicity, infinitely less philosophic than even the dualism of Descartes, since it has an infinity of ultimate principles instead of only two? If Spinoza sacrificed the individual to the universe, has not Leibniz, in his desire to emphasize the individual, gone to the other extreme? Apparently we are introduced to a universe that is a mere aggregate of an infinite multiplicity of realities, each independent of every other. Such a universe would not be a universe. It would be a chaos of disorder and conflict. We come, therefore, to a consideration of the relation between these individual monads and the universe. We have to discover what lifts the monads out of their isolation and bestows upon them that stamp of universality which makes it possible for them to enter into the coherent structure of reality: in a word, what is the universal content which the monad in its formal individuality bears and manifests?
The way in which the question has just been stated suggests the Leibnizian answer. The monad, indeed, in its form is thoroughly individual, having its own unique mode of activity; but its content, that which this activity manifests, is not peculiar to it as an individual, but is the substance or law of the universe. It is the very nature of the monad to be representative. Its activity consists in picturing or reproducing those relations which make up the world of reality. In a conscious soul, the ability thus to represent the world is called “perception,” and thus Leibniz attributes perception to all the monads. This is not to be understood as a conscious representation of reality to itself (for this the term “apperception” is reserved), but it signifies that the very essence of the monad is to produce states which are not its own peculiar possessions, but which reflect the facts and relations of the universe. Leibniz never wearies in finding new ways to express this purely representative character of the monad. The monads are little souls; they are mirrors of the world; they are concentrations of the universe, each expressing it in its own way; borrowing a term from scholasticism, they are “substantial forms.” They are substantial, for they are independent unities; they are forms, because the term “form” expresses, in Aristotelian phraseology, the type or law of some class of phenomena. The monad is an individual, but its whole content, its objectivity or reality, is the summation of the universe which it represents. It is individual, but whatever marks it as actual is some reproduction of the world. His reconciliation of the principles of individuality and universality is contained in the following words: “Each monad contains within itself an order corresponding to that of the universe,—indeed, the monads represent the universe in an infinity of ways, all different, and all true, thus multiplying the universe as many times as is possible, approaching the divine as near as may be, and giving the world all the perfection of which it is capable.” The monad is individual, for it represents reality in its own way, from its own point of view. It is universal, for its whole content is the order of the universe.
New light is thus thrown upon the former statement that reality is activity, that the measure of a being is the action which it puts forth. That statement is purely formal. It leaves the kind of activity and its law wholly undetermined. But this relation of “representativeness” which we have discovered gives definiteness. It is the law of the monad’s action to mirror, to reflect, the universe; its changes follow each other so as to bring about this reflection in the completest degree possible. The monad is literally the many in the one; it is the answer to the inquiry of Greek philosophy. The many are not present by way of participation in some underlying essence, not yet as statically possessed by the one, as attributes are sometimes supposed to inhere in a substratum. The “many” is the manifestation of the activity of the “one.” The one and the many are related as form and content in an organic unity, which is activity. The essence of a substance, says Leibniz, consists in that regular tendency of action by which its phenomena follow one another in a certain order; and that order, as he repeatedly states, is the order in which the universe itself is arranged.
The activity of a monad may be advantageously compared to that of a supposed atom, granting, for the sake of the illustration, that there is such a thing. Each is in a state of change: the atom changes its place, the monad its representation, and each in the simplest and most uniform way that its conditions permit. How, then, is there such a similarity, such a monotony, in the change of an atom, and such variety and complexity in the change of a monad? It is because the atom has merely parts, or external variety, while the monad has an internal variety. Multiplicity is organically wrought into its very being. It has an essential relation to all things in the universe; and to say that this relation is essential, is to say that it is one which constitutes its very content, its being. Hence the cause of the changes of the monad, of their variety and complexity, is one with the cause of the richness, the profusion, the regulated variety of change in the universe itself. While we have employed a comparison with atoms, this very comparison may serve to show us the impossibility of atoms as they are generally defined by the physicist turned philosopher. Atoms have no internal and essential relation to the world; they have no internal connection with any one thing in the world: and what is this but to say that they do not enter anywhere into the structure of the world? By their very conception they are