 Contrary to a now-common idea that the opposition between rationalism and empiricism should be replaced with a more historically legitimate opposition between experimental philosophy and speculative philosophy (Anstey 2005), Diderot’s case suggests a blend between empiricism cum experimentalism, and speculation. He is often confronted with the need to continue his analysis of phenomena beyond the limits of strict empiricism: the nature of matter, the limits of animation or on the more internal scale, the functioning of the nervous system or the mechanics of generation. And here the need for metaphysical imagination comes into play, which is not the same as a strictly abstract metaphysics. Once again, Diderot’s criticisms of mathematical abstraction in favor of the greater concreteness of the life sciences, which he shares with Buffon, can be adduced here.