The author presents a study of The Letter to Abba Simeon, a well-known pseudepigraph included in the collection of ascetical works of Isaac the Syrian (7th century). The work proves to be The Letter to Patrikios by the Western Syrian writer Philoxenos of Mabbog. Manuscript tradition of the interpolation and probable reasons for the false attribution are discussed. The latter were probably connected with the controversy over «Messalianism».