While the concept of functional differentiation is one of sociology’s oldest analytic tools, there is significant confusion about its meaning and purpose in the contemporary discipline. This article addresses one source of uncertainty: the conflicting array of ontological and methodological positions which are currently attached to the differentiation term. Drawing on Laudan’s philosophy of science, I argue that sociology does not house a unified program of differentiation theory, but is instead marked by at least two discrete traditions of differentiation thinking. (...) Disentangling these traditions will, I contend, clarify our thinking, and help reduce the prevalence of internally contradictory conceptual models. (shrink)