Here, for the first time in English, are the philosophical essays--including the first statement of the "Duhem Thesis"--that formed the basis for Aim and Structure of Physical Theory, together with new translations of the historiographical essays presenting the equally celebrated "Continuity Thesis" by Pierre Duhem (1861-1916), a founding figure of the history and philosophy of science. Prefaced by an introduction on Duhem's intellectual development and continuing significance, here as well are important subsequent essays in which Duhem elaborated key concpets and critiqued such contemporaries as Henri Poincaré and Ernst Mach. Together, these works offer a lively picture of the state of science at the turn of the twentieth century while addressing methodological issues that remain at the center of debate today.