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 etre
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.]

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 combinati