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 other hand bodies. The bodies are reducible to a certain number of solid corpuscles, indivisible, with differing forms, heavy and animated by an essential and spontaneous motion. These are the atoms which by their coming together constitute bodies.
Now it is evident that atoms in taking the place of other atoms, successively occupy in empty space places that are adequate to them, which have exactly the same extension and the same forms as the respective atoms. If at the moment when an atom is motionless in some place we imagine lines drawn following its contours (as when an object is being traced for transferring), is it not clear that if the atom were removed, we should have preserved its effigy, or a sort of silhouette, its geometric form upon a foundation of empty space? We should obtain thus a portion of space, which I will call an empty atom, in contrast with the full atom which was there before.
Now I ask the atomists to explain what distinguishes the full atom from the empty one, what are the characteristics that may be found in one and not in the other. Is it the being extended? No, for the empty atom is extended like the full atom. Is it the having a form? No, for the empty atom has a form as has the full atom and exactly the same form. Is it the being indivisible? No, for it is still more difficult to understand the divisibility of space than of the body. In a word everything which depends on extension is the same in the empty atom as in the full atom. But the empty atom is not a body and contains nothing corporeal; therefore extension is not the essence of bodies and perhaps does not constitute a part of this essence. May we say that it is the motion which distinguishes the full atom from the empty atom? But before beginning to move the atom must have already been something, because that which is nothing in itself can be neither at rest, nor in motion. Motion, therefore, is a dependent and subordinate phenomenon which already presupposes a defined essence. If we examine carefully we will see that what really distinguishes the full atom from the empty atom is its solidity or weight. Neither solidity nor weight, however, are modifications of extension; both come from force. It is accordingly, force and not extension which constitutes the essence of the body.
Turning now to those who, like the Cartesians, are unwilling to admit the possibility of a vacuum and maintain that all space is full, the demonstration is still more simple, for we may ask in what filled space, taken in its entirety, differs from empty space taken in its entirety. Both are infinite; both are ideally divisible and both are really indivisible; both are susceptible of modalities in form or of geometrically defined forms. Perhaps it will be claimed that in full space the particles are movable and can supplant one another; in this case we are back in the preceding line of argument and we shall ask in what these movable particles are distinguished from the immovable particles of space among which they move. Thus the Cartesians, like the atomists, will be obliged to recognize that the plenum is distinguished from the vacuum only by resistance, solidity, motion, activity, in a word, force.
To those who reproach the Leibnizian conception with idealizing matter too much, it may be replied that matter taken in itself is necessarily ideal and super-sensible. Of course it cannot be said that a body is only an assembly of subjective modifications. The Berkeleyan idealism is a superficial idealism, which will not stand examination; for when I shall have reduced the whole universe to a dream of my mind and to an expansion of myself the question will still remain whence comes this my dream and what are the causes which have produced in me so complicated a hallucination; these causes are outside of me and they go beyond me on every side; it would therefore be very inappropriate for me to call them myself, for the I is strictly that of which I have consciousness. The Fichtean Ich, which by reaction against itself thus produces the nicht-ich is only a complicated and artificial circumlocution for saying in a paradoxical form that there is a not-I. At most, we can conjecture with the absolute idealism that the I and the not-I are only two faces of one and the same being, which involves them both in an infinite activity; but we thus reach a position very far from the idealism of Berkeley.
To return to the idealism of Leibniz, I think it can be shown a priori that matter taken in itself is something ideal and super-sensible, at least to those who admit a divine intelligence. For it will readily be granted that God does not know matter by means of the senses; for it is an axiom in metaphysics that God has no senses and consequently cannot have sensations. Thus: God can be neither warm nor cold; he cannot smell the odor of flowers; he cannot hear sounds, he cannot see colors; he cannot feel electrical disturbances, etc. In a word, since he is a pure intelligence he can conceive only the purely intelligible; not that he is ignorant of any of the phenomena of nature, only that he knows them in their intelligible reasons and not through their sensible impressions, by means of which creatures are aware of them. Sensibility supposes a subject with senses, organs and nerves, that is, it is a relation between created things. From God’s point of view, therefore, matter is not sensible; it is, as the Germans say, übersinnlich. The conclusion is easy to draw, namely, that God, being absolute intelligence, necessarily sees things as they are, and conversely the things in themselves are such as he sees them. Matter is, accordingly, such in itself as God sees it, but he sees it only in its ideal and intelligible essence; whence we see that matter is an intelligible something and not something sensible.
To be sure we may not conclude from this point that the essence of matter does not consist in extension, for it could be maintained that extension is an object of pure intelligence quite as well as force. But without taking up the difficulty of disengaging extension from every sensible element, I wish to establish only one thing, namely that Leibniz cannot be reproached with idealizing matter, since this must be done in every system, at least in those which admit a divine logos and a foreordaining reason.
One of the most widely spread objections against the monadological system is the impossibility of composing an extended whole out of non-extended elements. This is Euler’s principal objection in one of his Letters to a German Princess and he considered it absolutely definitive because the necessary consequence of such a system would be to deny the reality of extension and of space, and to launch out thus into all the difficulties of the idealistic labyrinth. I think,
