Abstract: The work of Joseph Maréchal that we analyse in this essay, is a complete metaphysical theory of knowledge. The main text that we discuss, constitutes the end point of a long process of historiographical enquiry, which culminates in an exegetical exposition of the doctrine of the knowledge of Thomas Aquinas, seen and interpreted in a continuous relationship with Criticism. In the first section of this essay, we will follow the path of an a posteriori psychological analysis of human knowledge. The second section examines Maréchal’s interpretation of the Kantian Transcendental Deduction of the Categories. The two different sections of this paper follows a substantially symmetrical pattern: in the first part we retrace the main doctrines of the Thomistic ontology of knowledge, according Maréchal’s synthesis, and this by a recognition of the theory of perception, and then moving towards an abstractive account of the concept’s genesis, and ending with an analysis of the judgment and its formal structure. The second section, symmetrically, studies the synthetic genesis of phenomenal datum, the deductive genesis of categories and the transcendental doctrine of judgment.