The problem is to invent what has recently been nicknamed “outdoor psychology” (Geertz 1983). The book is an inquiry into conditions that would make this possible. The conclusion: that contemporary theorizing about social practice offers a means of exit from a theoretical perspective that depends upon a claustrophobic view of cognition from inside the laboratory and school. The project is a “social anthropology of cognition” rather than a “psychology” because there is reason to suspect that what we call cognition is in fact a complex social phenomenon. The point is not so much that arrangements of knowledge in the head correspond in a complicated way to the social world outside the head, but that they are socially organized in such a fashion as to be indivisible. “Cognition” observed in everyday practice is distributed – stretched over, not divided among — mind, body, activity and culturally organized settings (which include other actors). Empirical support for this proposal has emerged recently from research exploring the practice of mathematics in a variety of common settings. These studies converge towards a view that math “activity” (to propose a term for a distributed form of cognition) takes form differently in different situations. The specificity of arithmetic practice within a situation, and discontinuities between situations, constitute a provisional basis for pursuing explanations of cognition as a nexus of relations between the mind at work and the world in which it works.