him, no substance is, properly speaking, passive. Passion in a substance is nothing else than an action considered bound to another action in another substance. Every substance acts only through itself and cannot act upon any other. The monads have no windows through which to receive anything from outside. They do not undergo any action and consequently are never passive. All that takes place in them is the spontaneous development of their own essence. All that there is, is that the states of each one correspond to the states of all the others. When we consider one of these states in one monad as corresponding to a certain other state in another monad, in such a way that the latter is the condition of the former, the first state is called a passion and the second an action. There is, therefore, between all monad-substances a pre-established harmony, in accordance with which each one represents (or expresses, as Leibniz says) the whole universe. But this is ever only the development of its own activity.
In restoring to created substances the activity which the Cartesian school had too much sacrificed, Leibniz thought to contribute to the clearer distinction between the created and the Creator. He justly remarked that the more the activity of the created things is diminished, the more necessary becomes the intervention of God, in such a way that if all activity in created things is suppressed, then we must say that it is God who brings everything in them to pass and who is at the same time their being and their action (operari et esse). What difference, however, is there between this point of view and that of Spinoza? Would we not thus make nature the life and the development of the divine nature? In fact, by this hypothesis, nature is reduced to a mass of modes of which God is the substance. He, therefore, is all that there is of reality in bodies as well as in spirits.
To these five fundamental reasons given by Leibniz it will perhaps be allowed us to add a few particular considerations.
Those who deny that the essence of bodies is only in force, either admit the vacuum with the atomists, ancient and modern, or else like the Cartesians they do not admit it. Let us take up each of these positions separately.
For the atomists, disciples of Democritus and of Epicurus, or of Gassendi, the universe is composed of two elements, the vacuum and the plenum, on the one hand space and on the
