  Moreover, she even uses her PSR to establish other claims about material substance, claims that she also takes to be necessary in the same sense of necessity that applies to simple beings (i.e. what we may call "hypothetical necessity" above). In using the PSR, we observe the phenomenal facts of our actual world, and we can then posit hypotheses about what must further be true of substance in order to make those facts   possible.[11]  We see this method at work in several sections of the Institutions; for example, monads are active, she concludes, because this would explain the brute fact of motion in the phenomenal world, and she goes so far as to claim that force is also necessary to the nature of matter (IP §139). At the same time, precisely because things do not always move in the natural world, this force must be of two kinds: active force—the source of motion—and passive force, or inertia—the source of rest (IP §142). From her letter to Maupertuis of 30 April 1738 (#122), we know that she was familiar with Leibniz's work on dynamics and metaphysics, having sought out his articles on the topics—articles such as “The Brief Demonstration of the Error of Descartes” and “A Specimen of Dynamics”. So again, phenomena of our actual world that Leibniz details in those papers lead Du Châtelet to the same conclusions regarding the metaphysics of substance that Leibniz reaches—namely, that the unextended simple substances are also internally active, as well as constrained in their activity, through their possession of active and passive force, as this claim helps explain the phenomena of the natural  world.[12]