ENTwo outstanding Polish historians of the 20th c. shared interest in the history of Poland and Eastern Europe, research skills and erudition, as well as patriotism. What divided them, at least initially, were the master-student relationship, seven years' age difference, and research temperament. The objects of their interest included Rus' and Lithuania - firstly, as objects of historical geography, and secondly, as political (state) organisms. Oskar Halecki, whose road to the history of Eastern Europe led through research into the Polish-Lithuanian Union, started with defining the terms of medieval Lithuania and Rus' in political sense, to capture them years later within the framework of a broad historiosophical synthesis of the notion of Europe. Also Henryk Paszkiewicz defined the historical notions of Rus' and Lithuania, but he did so summing up his previous research. In a comprehensive three-volumed is quisition on Rus', Paszkiewicz questioned the existence of statehood and cultural continuity between the early medieval Kievan Rus' and the subsequent Vladimir-Suzdal Rus' which evolved into the Russian Empire. He emphasized clear Finno-Ugric and Mongolian influences in the formation of the ethnicity and political traditions of the Moscow state and its successor, Russia.While expressed differently and put forward through different channels, the opinions of both the master and the student on Eastern Europe (in cultural / civilization terms) are similar. Paszkiewicz' s work consolidated Halecki's opinion on the history of the peoples of Central and Eastern Europe. The latter agreed with his student's view on the nature of the Moscow state, which followed Mongolian political and social patterns. Halecki and Paszkiewicz also shared a view of the genesis of the Lithuanian state and its expansion to the Ruthenian territories. Owing to both researchers' similar views on the federal character of the Lithuanian state, the genesis of the Polish-Lithuanian union, and the perception of differences - cultural, political, and ethnic - between Vladimir-Suzdal Rus' and Western Rus', Paszkiewicz readily subscribed to the so-called Jagiellonian idea expressed by Halecki. Just like Paszkiewicz, who fully accepted Halecki 's view on the history of the union and the place of Rus 'in the Polish-Lithuanian state, the latter also accepted his former student's and friend's proposals as being inviolable and relied on them in designing historiosophical constructs.