When Leibniz characterizes substantial form as "primitive active force" and pri-
mary matter as "primitive passive force" ¹ the word 'primitive' marks a contrast,
sometimes explicit, with "derivative" active and passive forces. The forces stud-
ied in physics are classifed as derivative by Leibniz. And we confront grave dif-
ficulties in trying to understand what relationship he envisaged between the
derivative physical forces and the primitive forces that constitute, metaphysically,
the essence of substance. These difficulties are probably the largest obstacle to
understanding the relation between Leibnizian physics and Leibnizian metaphys-
ics. They are the topic of the present chapter.

Leibniz clearly thought, as his terminology indicates, that the derivative forces
are derived from the primitive forces, and there are many texts from Leibniz's
middle years in which he speaks of derivative forces more specifically as modi-
fications of primitive forces. ² In some of them he explicitly invokes the analogy
of the relation of shape to extension, making it clear that he has in mind some-
thing like the relation that obtains between mode and attribute in Descartes's
thought. On 22 March 1703, for example, he wrote to Isaac Jaquelot that

the derivative or accidental force that cannot be denied to bodies in motion
must be a modification of the primitive [force], as shape is a modification of
extension. Accidental forces cannot occur in a substance without essential
force, for accidents are nothing but modifications or limitations, and cannot
contain more perfection or reality than the substance. ( G III,457) ³

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