Mercier, a founder of a Neo-scholastic Louvain school, showing not only the issues crucial to his philosophy, but also an evolution of an epistemological thought by a Belgian cardinal, based on particular editions of his works and a rich philosophical output. It concerns mainly A general criteriology, Mercier’s main work in which a systematic theory of cognition is derived from Thomas Aquinas’s classical texts, confirming a tomistic epistemology on the paths of a philosophical realism. In its main part, the work consists of the introduction, five chapters and conclusion. The first chapter depicts a historical background against which Mercier’s conception developed, and focuses mainly on the presentation of the programme and the method of an academic activity conducted in the Higher Institute of Philosophy founded in Louvain by Mercier. The second chapter examines an evolution of Mercier’s means of interpretation of a classical definition of truth, and leaves a place for the analysis of three fundamental issues (the three subsequent chapters) which, according to Mercier, define a proper position of a criteriological problem. The first of them (in chapter three) is related to the attitude an intellect should hold towards the issue of certainty. Mercier’s point of view, regarded as “a rational dogmatism”, is presented in a discussion with the advocates of scepticism and Cartesian standpoint on the one hand, and the representatives of the so called “old dogmatism” on the other. The second issue (chapter four) concerns the nature of the references in his opinion between its two basic constituents: a subject and predicate, and, thus, it is connected with the first fundamental problem — a synthesis of the subject and predicate, i.e. the question on the status and cognitive value of different kinds of opinion. Before Mercier solves the problem, he polemicises with Immanuel Kant’s position, as well as main theses of a positivist philosophy. Such a polemics, in the case of Kant, relates to the issue of an objective certainty of academic opinions, whereas in the case of positivists, their objective and essential nature. Finally, the third aspect (chapter five) is connected with the problem of an ontic status of notions constituting the subject and predicate in a statement, and, thus, concerns the so called second fundamental issue, i.e. the issue of an objective reality of notions. In his analysis, Mercier attempts to show that cognitive forms giving statements to predicates, possess an objective reality, however, the way he justifies his thesis becomes the subject of a serious philosophical argument. The two most important voices in this argument are represented by Étienne Gilson, a creator of the conception of a methodological realism on the one hand, and Léon Noël, Mercier’s student, in favour of a critical direct realism on the other hand. As much as Gilson tries to prove that Mercier’s standpoint is eventually an indirect realism, and, in consequence, an illationism of a Cartesian type, Noël wants to prove that Mercier is still in a position of a critical direct realism and his conception is based not on illation, but immediatism. A critical analysis of Mercier’s conception shows that it is Noël’s point of view that is more probable. Mercier’s general theory of certainty is definitely realistic and can be valuable in a discussion with not only contemporary versions of idealism, but also with different branches of a realistic philosophy