The purpose of the article is to show the genesis of the “Jagiellonian idea,” which the author links to the political and social changes in medieval Poland of the Piast era. Ecclesiastical law, which was associated with close relations with the Holy See and Rome, exerted a great influence on the formation of the state system of that time. Under the influence of these ties, the social system in Poland took shape somewhat differently than in other regions of Europe that were dominated by absolutism and had autocratic rulers. Under the law of the time, all members of society, despite class differences, were bound by the same ethical and moral principles. After the Jagiellonians took over the reign, the system evolved into a mixed monarchy, in which power was shared by three parliamentary branches: the king, the senate and the chamber of deputies.