This paper seeks to defend, develop, and revise Edward Craig's “genealogy of knowledge”. The paper first develops the suggestion that Craig's project is naturally thought of as an important instance of “social cognitive ecology”. It then introduces the genealogy of knowledge and some of its main problems and weaknesses, suggesting that these are best taken as challenges for further work rather than as refutations. The central sections of the paper conduct a critical dialogue between Craig's theory and Wittgenstein's claim–familiar from On Certainty–that common-sense certainties cannot be known. It turns out that Craig's distinction between different stages in the development of our concept of knowledge can illuminate and make plausible Wittgenstein's claim. But it can do so only if Craig's traditional commitment to a central “core” in our concept of knowledge is replaced with the idea of knowledge as a family-resemblance concept.