1. THE DOCTRINE OF MONADS.—The fundamental peculiarity of Leibnitz’s theory is its opposition to Spinozism. Substance, as the indeterminate universal, was with Spinoza the only positive. With Leibnitz also the conception of substance lay at the basis of his philosophy, but his definition of it was entirely different. While Spinoza had sought to exclude from his substance every positive determination, and especially all acting, and had apprehended it simply as pure being, Leibnitz viewed it as living activity and active energy, an example for which might be found in a stretched bow, which moved and straightened itself through its own energy as soon as the external hindrance was removed. That this active energy forms the essence of substance is a principle to which Leibnitz ever returns, and from which, in fact, all the other chief points in his philosophy may be derived. From this there follow at the outset two determinations of substance directly opposed to Spinozism; first, that it is a single being, a monad; and second, that there are a multiplicity of monads. The first follows because substance, in so far as it exercises an activity similar to an elastic body, is essentially an excluding activity, or repulsion; the conception of an individual or a monad being that which excludes another from itself. The second follows because the existence of one monad involves the existence of many. The conception of one individual postulates other individuals, which stand over against the one as excluded from it. Hence the fundamental thesis of the Leibnitz philosophy in opposition to Spinozism is this, viz., there is a multiplicity of individual substances or monads.
2. THE MONADS MORE ACCURATELY DETERMINED.—The monads of Leibnitz are similar to atoms in their general features. Like these they are corpuscular units, independent of any external influence, and indestructible by any external power. But notwithstanding this similarity, there is an important and characteristic difference between the two. First, the atoms are not distinguished from each other, they are all qualitatively alike; but each one of the monads is different in quality from every other, every one is a peculiar world for itself, every one is different from every other. According to Leibnitz, there are no two things in the world which are exactly alike. Secondly, atoms can be considered as extended and divisible, but the monads are metaphysical points, and actually indivisible. Here, lest we should stumble at this proposition (for an aggregate of unextended monads can never give an extended world), we must take into consideration Leibnitz’s view of space, which, according to him, is not something real, but only confused, subjective representation. Thirdly, the monad is a representative being. With the atomists such a determination would amount to nothing, but with Leibnitz it has a very important part to play. According to him, in every monad, every other is reflected; every monad is a living mirror of the universe, and ideally contains the whole within itself as in a germ. In thus mirroring the world, however, the monad is not passive but spontaneously self-active: it does not receive the images which it mirrors, but produces them spontaneously itself, as the soul does a dream. In every monad, therefore, the all-seeing and all-knowing one might read every thing, even the future, since this is potentially contained in the present. Every monad is a kind of God. (_Parvus in suo genere Deus._)