Search for biblical ancestors of the Slavs, the so-called Letter of Alexander the Great on granting the Slavs the rights to the territories they occupy, the story of the three "Croatian brothers" Cech, Leh and Ruse - the origin and history of these and a number of other legends related to them constitute the main content of the monograph A. S. Mylnikov. Analyzing the process of searching for scientific truth, which took place in the struggle of the rationalistic principle with mythologized consciousness, the author shows that the ethnohistoric legends analyzed by him were based on the idea of ​ ​ the ethnogenetic and linguistic kinship of the Slavic peoples. With the help of a wide range of monuments of historical thought in Slavic, Latin, German and other languages, the author reconstructs one of the important aspects of the ethnocultural system of the Slavic world of the early New Age (from the turn of the 15th - 16th centuries to the beginning of the 18th century).