 Diderot is rather unexpectedly combining Spinoza’s metaphysics of substance with a new theory of biological development, epigenesis, according to which the embryo grows by the successive addition of layers of purely material substance. Why call the latter view “modern Spinozism”? The “ancient Spinozists” are substance monists and metaphysicians, while their modern descendants are also committed to biological epigenesis, and assert that matter is fundamentally living matter. Is this Spinozism or not? What possible relation could there be between Spinozism and epigenesis? Or how can a metaphysics of substance and modes, which says almost nothing about biological entities even if it is also a major statement of philosophical naturalism, also be a fashionable embryological theory of the seventeenth and eighteenth centuries? In fact, very few commentators have asked why Diderot gives such an idiosyncratic definition of “modern Spinozism”.