In this article I will analyze the central categories of Thomas Aquinas’s social thought, such as a people (populus), multitude (multitudo), and Commonwealth (respublica). The next article (Part 2) will contain an investigation of the categories of a community (communitas), communication, and society (societas). I stress the immediate readiness of the question in existing Thomistic literature. Despite the active investigations of Aquinas’s political theory, the social theory remains almost forgotten. The works of Ignatius Th. Eschmann, Yves Congar, and Jeremy Catto represent some exclusions from this assertion, but not one of them has paid enough attention to the terminological peculiarities of Thomistic thought. Between the main results of this work, it is worth to focus on the next aspects of the dissipation of the people’s concept, its equalization with the multitude, and the break of the connection between the notions of a peo-ple and a Commonwealth. The populus in Thomas’s theory loses its political nature ascribed to it by Cicero and Au-gustin. Having lost its subjectivity, the People convert into an organized multitude united by common territory and the same mode of every-day life. Aquinas ignores the creation of the Commonwealth by the People and establishes a con-nection of another type between these concepts. According to him, the People is a kind of Aristotelian “materia,” while the Commonwealth is the “form.” In compliance with the precedential assertion, the Respublica becomes eternal and unchangeable, where only the content—i.e., the People or the multitude—can change. In effect, Aquinas formulates the concept of the proto-State here.