opposes to it, and this is what he calls inertia. It is evident, therefore that if a substance in repose reveals itself by its resistance to motion, the argument of the first mover, far from being weakened is, on the contrary, strengthened.
Besides this, even if a spontaneous disposition to movement, be admitted in the elements of bodies, yet experience compels us to recognize that this disposition passes over into action only upon the excitation of an exterior action because we never see a body put in motion except in the presence of another. The actual indifference to movement and to repose, which at the present time is called, in mechanics, inertia, must always be admitted, whether we posit in the body a virtual disposition to movement or whether, on the contrary, the body be considered as absolutely passive; in either case there must be a cause determining the motion; it is not necessary that this first cause produce everything in the body moved, and that it should be in some sort the total cause of the motion; sufficient is it for it to be the complementary cause as the Schoolmen used to say.
Furthermore inertia must not be confounded with absolute inactivity. Leibniz showed admirably that an absolutely passive substance would be a pure nothing; that a being is active in proportion as it is in existence; in a word, that to be and to act are one and the same thing. From the fact, however, that a substance is essentially active, it does not necessarily follow that it is endowed with spontaneous motion, for the latter is only a special mode of activity and is not the only one. For example, resistance, or impenetrability, is a certain kind of activity, but is not motion. They are mistaken, therefore, who think that the theory of active matter does away with a first cause for motion, because even if motion be essential to matter, we will still have to explain why no portion of matter is ever spontaneously in motion.
In short, according to Leibniz, every being is essentially active. That which does not act does not exist; quid non agit non existit. Now, whatever acts is force; therefore, everything is force or a compound of forces. The essence of matter is not, as Descartes thought, inert extension, it is action, effort, energy. Furthermore the body is a compound and the compound presupposes a simple. The forces, therefore, which compose the body are simple elements, unextended—incorporeal atoms. Thus the universe is a vast dynamism, a wise
