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.]

The atomists are right when they postulate for the explanation of phenomenal bodies simple, indivisible, eternal units, for every composite consists of simple parts.  But they are wrong when they regard these invisible, minute corpuscles, which are intended to subserve this purpose as indivisible:  everything that is material, however small it be, is divisible to infinity, nay, is in fact endlessly divided.  If we are to find indivisible units, we must pass over into the realm of the immaterial and come to the conclusion that bodies are composed of immaterial constituents.  Physical points, the atoms, are physical, but not points; mathematical points are indivisible, but not real; metaphysical or substantial points, the incorporeal, soul-like units, alone combine in themselves indivisibility and reality—­the monads are the true atoms.  Together with indivisibility they possess immortality; as it is impossible for them to arise and perish through the combination and separation of parts, they cannot come into being or pass out of it in any natural way whatever, but only by creation or annihilation.  Their non-spatial or punctual character implies the impossibility of all external influence, the monad develops its states from its own inner nature, has need of no other thing, is sufficient unto itself, and therefore deserves the Aristotelian name, entelechy.

Thus two lines of thought combine in the concept of the monad.  Gratefully recognizing the suggestions from both sides, Leibnitz called Cartesianism the antechamber of the true philosophy, and atomism the preparation for the theory of monads.  From the first it followed that the substances were self-acting forces; from the second, that they were immaterial units.  Through the combination of both determinations we gain information concerning the kind of force or activity which constitutes the being of the monad:  the monads are representative forces.  There is nothing truly real in the world save the monads and their representations [ideas, perceptions].