This article is an introduction to the Russian translation of the first book of Thomas Aquinas’ treatise De regimine principum. The author considers the place of the text within the framework of the European tradition of the Mirrors of Princes, while describing, in brief, the problems of the authorship and the dating of the treatise. Among the European Mirrors of the Princes, the work of Thomas Aquinas, On Kingship or On the Government of Princes, has a special place. Thanks to its author’s reputation, this text became one of the most famous influences in both European Late Medieval Philosophy and Modern Political Philosophy. Additionally, this treatise has become a model for two famous works of the same name, On the Government of Princes, written by Ptolemy of Lucca, and Egidio Colonna. In the discussion of the dating of Aquinas’ book, the author holds the opinion that this work was composed between 1271 and 1273, and was addressed to Hugh III Lusignan, the king of Cyprus. The special place in this article is occupied by a small terminological discussion of the Russian translation of the Latin word “princeps.” The author affirms that the existing translation of this word as “Lord” (gosudar) is impossible and quite incorrect. In the author's opinion, the correct translation is “the ruler,” or “the Prince.”