The thesis offers an exposition and tentative solution of two problems: a definition of the Idea of Progress, and classification of social philosophers and sociologists according to this definition. Twelve propositions, or assumptions, are used to define the Idea, and works of selected philosophers from Hesiod to Parsons are examined in the light of this definition. Historical examination reveals that the Idea's acceptance reaches a zenith in the early nineteenth century, after which it lost credibility, becoming virtually discarded by mid-twentieth century.