I begin by offering a brief definition of the term “rationalism”, first in its strictly philosophical sense as denoting preference for the use of reason over other means of acquiring knowledge, and then in its broader sense as a term denoting patient reasoning, scepticism and reappraisal of received opinions. In this latter sense rationalism informed the critical movement in seventeenth- and eighteenth-century European thought which challenged dogmatic theology, weakened ecclesiastical power and secularized culture. Next, I briefly examine the currency and characteristics of Russian rationalism, particularly as the radical intelligentsia of the 1860s understood the concept (which they often described as “realism”). I then turn to the work of Petr Tkachev, who is chiefly remembered as an émigré revolutionary strategist of the 1870s, an advocate of seizure of political power by a conspiratorial minority and therefore a possible forerunner of Lenin, but who was also a prolific writer on social and philosophical matters before he was arrested and imprisoned in 1869 in connection with the Nechaev affair. I focus on an article, “Essays from the History of Rationalism” and associated fragments, which were seized by the police in 1866 and which were not published in Tkachev's lifetime. Tkachev's article describes and discusses the history of religious dogmatism and persecution in medieval and early-modern Europe. Scholars have often taken it to be a significant original work. In fact, I claim, it is heavily dependent on a contemporaneous book, the History of the Rise and Influence of the Spirit of Rationalism written by the Irish historian William Lecky. Tkachev did acknowledge his debt to Lecky, but not the extent or nature of the debt; indeed he distanced himself from Lecky. And yet he not only plagiarized the greater part of his article from Lecky (who wrote far more elegantly than Tkachev himself) but also drew from Lecky, no less than from Marx, his thesis that economic considerations underlie ideas and values, as well as the dense factual information for which he admitted Lecky was his source. In this respect Tkachev's article illustrates the derivative nature of Russian rationalism and also indicates the extent to which British influence was overtaking French and German influence in the radical intelligentsia in the 1860s. At the same time, Tkachev's “rationalism” lacks the broad humanism and toleration that infuses the spirit of rationalism as Lecky conceives of it. His article thus reveals the transformation and impoverishment of a European body of thought when it was transplanted to Russian soil and exemplifies features of the cast of mind of the pre-revolutionary Russian radical intelligentsia on which Sergei Bulgakov remarked in a passage in his contribution to the Landmarks volume that I have taken as my starting point.