Having completed our reconstruction of the trajectory of the young Descartes, from physico-mathematician, in 1619, to systematic mechanistic natural philosopher, in 1633, one more step is required to round off our inquiry. His Principia Philosophiae contains the mature statement of his system of natural philosophy. Therefore, our study of how Descartes matured as a natural philosopher can only be properly closed through an analysis of the Principia and comparison of it to Le Monde. The present Chapter explicates previously little noticed, but daring and masterful new moves in pro-Copernican systematization that Descartes executed in the Principia. Descartes' systematizing strategy focused upon weaving ranges of novel matters of fact—concerning sunspots, novae and variable stars, and the structure and formation of all planets—into explanatory and descriptive "cosmographic" narratives with cosmic sweep and radical realist Copernican intent. It is this vast system–binding gambit of Descartes, entraining the use and reframing of key, available matters of fact—in turn leveraged into explanatory resources within the system—that best characterises the natural philosophical difference between Le Monde and the Principia.