‘Spencer is dead’, wrote Talcott Parsons at the beginning of The Structureof Social Action, ‘but who killed him and how ? This is the problem’. In this study, which was both the foundation of Parsons’ structural-functionalism and a major reinterpretation of the history of modern social science, Spencer stood for a vanquished schoolof social thought. He represented positivism at the suicidal extreme where its naive individualism fell apart, paradoxically passing over into its antithesis, a biological determinism precluding individual initiative. His thought had died at the intersection of individual and society.Beyond this point, Parsons discovered the rise of a new social theory in Marshall, Pareto, Durkheim and Weber. From these four thinkers, working independently of one another, Parsons tried to put together the pieces of a system, succeeding where Spencer had decisively failed, reconciling personal agency and social order.