In the past decade, several Descartes scholars have gone on record claiming that, for biological purposes, Descartes likely accepts the practical scientific necessity of the existence of “physical natures,” even while his official substance-mode ontology and his characterization of matter in terms of extension do not license the existence of physical natures. In this article, I elaborate on the historical context of Descartes’s biology, the “practical scientific necessity” just mentioned, and argue, contrary to other interpretations, that Descartes does offer a (...) philosophical justification for the existence of physical natures, albeit not by appeal to the categories of substance and mode, or matter as such, but by appeal to our “nature” as a union between mind and body. (shrink)