Leibnitz develops his new concept of substance, the monad,[1] in conjunction with, yet in opposition to, the Cartesian and the atomistic conceptions. The Cartesians are right when they make the concept of substance the cardinal point in metaphysics and explain it by the concept of independence. But they are wrong in their further definition of this second concept. If we take independence in the sense of unlimitedness and aseity, we can speak, as the example of Spinoza shows, of only one, the divine substance. If the Spinozistic result is to be avoided, we must substitute independent action for independent existence, self-activity for self-existence. Substance is not that which exists through itself (otherwise there would be no finite substances), but that which acts through itself, or that which contains in itself the ground of its changing states. Substance is to be defined by active force,[2] by which we mean something different from and better than the bare possibility or capacity of the Scholastics. The _potentia sive facultas_, in order to issue into action, requires positive stimulation from without, while the _vis activa_ (like an elastic body) sets itself in motion whenever no external hindrance opposes. Substance is a being capable of action (_la substance est un être capable d'action_). With the equation of activity and existence (_quod non agit, non existit_) the substantiality which Spinoza had taken away from individual things is restored to them: they are active, consequently, in spite of their limitedness, substantial beings (_quod agit, est substantia singularis_). Because of its inner activity every existing thing is a determinate individual, and different from every other being. Substance is an individual being endowed with force. [Footnote 1: According to L. Stein's conjecture, Leibnitz took the expression Monad, which he employs after 1696, from the younger (Franc. Mercurius) van Helmont.] [Footnote 2: Francis Glisson (1596-1677, professor of medicine in Cambridge and London) had as early as 1671, conceived substances as forces in his treatise _De Natura Substantiae Energetica_. That Glisson influenced Leibnitz, as maintained by H. Marion (Paris, 1880), has not been proven; cf. L. Stein, p. 184.]