Much of the controversy surrounding the correspondence traditionally attributed to Prince Andrei M. Kurbskii and Tsar Ivan Groznyi in 156479 has centered on the dating of various texts that bear a remarkable similarity to passages in the correspondence. In his criticism of the traditional attribution, Professor Keenan has argued that the author of KI (the first letter attributed to Kurbskii), which was supposedly composed in Lithuania in 1564, borrowed key phrases from the Complaint of the obscure monk Isaiah, a document apparently written in 1566.1 This debate has remained inconclusive, however, owing to the fact that such similarities among texts do not clearly prove which served as the source for the other. Even if a resemblance is demonstrated, the direction of borrowing must also be determined. Several scholars have argued that the similarities between Isaiah's Complaint and KI may be explained by the possibility that Isaiah saw the letter and borrowed from it, rather than the reverse.2 In his comparisons of various texts to passages in KI, Keenan has been hampered by the fact that the letter does not identify the passages in question as having been quoted from another source. The hypothesis of textual dependence rests on tenuous lexical and stylistic evidence, without any verifiable attribution. Although some of the similarities are indeed striking, Keenan's critics have resisted his conclusion, based on textual and other evidence, such as watermarks, that the entire correspondence was composed after 1620.