 Diderot’s natural philosophy is deeply and centrally “biologistic”. As it emerges in the mid-eighteenth century, at a time before the appearance of the term “biology” as a way of designating a unified science of life, his project is motivated by the desire both to understand the laws governing organic beings and to emphasize, more philosophically, the uniqueness of organic beings within the physical world as a whole. Consider a little-known aspect of Diderot’s articulation of his project: his statement in favour of biological epigenesis within his short entry “Spinosiste” in the Encyclopédie. The entry does not bear his name, but large parts of the content occur elsewhere in his writings, and it is included in all editions of his works. Here he grafts new biological ideas such as epigenesis onto a Spinozist substance metaphysics (Wolfe 2014a), distinguishing between “ancient” and “modern” Spinozists and emphasizing that the latter specifically hold that “matter is sensitive”, as demonstrated “by the development of the egg, an inert body which by means of heat alone moves to the state of a sensing, living being”. For modern Spinozists, “only matter exists, and that it is sufficient to explain everything. For the rest, they follow ancient Spinozism in all of its consequences” (Enc. XV: 474).