The theories of a necessary progress and of a therapeutic science of society both appeared upon the scene. They were to result in a new split between political theory and political practice; a predilection to psychological explanation, and an antagonism to historical and philosophical explanation. They strengthened rather than created many existing attitudes that we will find systematized to a unique degree in subsequent American social thought. They were to furnish added conditions, in turn, for the emergence of a unique type of philosophy, pragmatism, and of a largely unsystematized but increasingly influential positivism. From the debates that attended the reception of Herbert Spencer’s thought and from a social gloss on Darwinian biology, progress and social science jointly emerged as a rational structure fulfilling an immanence of already existing American sentiments. From this time on, ‘Progress’ and ‘Science’ become the master-concepts of a distinctively American social thought. It is impossible to understand the grounds for a ‘science of politics’ without studying the reception of the doctrine of Progress, even though its first appearance, strictly speaking, was in sociological theory. Later we will show that the American political scientists never reflected upon the meaning of the doctrine of Progress: they merely accepted it as an established part of American thought, showing by their own actions how deeply it was established, but how little understood. It is another case of the importance of popular thought in American intellectual development.