This article examines the connections among heuristics, the epistemological and ontological presuppositions that underlie theorizing, and substantive explanations in sociology. It develops and contrasts three heuristics: “doing as knowing” (DK), “categorizing as knowing” (CK), and “praxis as knowing” (PK). These are each composed of four dimensions: the theory of knowledge, the theory of reality, the theory of the growth of knowledge, and the theory of knowledge producers. The article then shows the importance of heuristics for empirical work by demonstrating how (...) they shape explanations in the sociological subfield of the historical sociology of knowledge. The essay draws two main conclusions: it argues that PK offers a more useful basis for developing explanations in sociology than either of the two alternatives (DK and CK) that currently shape substantive work; furthermore, it claims that the exposition of heuristic assumptions is an important task for sociology. (shrink)