The question “Did Peter the Great exert a beneficial or a harmful influence upon the development of Russian history and culture?” is one that has provoked debate ever since that monarch ruled Russia in the first quarter of the eighteenth century. The varied answers to that question set up the general battle lines between Westernizers and Slavophiles during the first half of the nineteenth century. One of Russia's most critical intellectuals in that period, Peter Iakovlevich Chaadaev, answered the question in several ways. His answers provide a key to a peculiar kind of ideological westernization in Russian intellectual history. The discovery of hitherto unpublished source materials that fill an important gap in the documentation of Chaadaev's views on Peter the Great led to the writing of this article.