Problematization. In psychology, there is a situation of many working ontologies, expanding their boundaries to a general ontology. Accordingly, we have different explanations of the psyche and many methods of its development. In reality, the psyche is one. Research objectives. Substantiation of the method of studying the integrity of the psyche using the "One Psychology" analysis of the anthropological project of L. S. Vygotsky and the critical ontology of N. Hartman. Methodology. In the study of the psyche as a whole the post-non-classical mode of rationality makes it possible to rely on a fundamental ontology, Hence, we used in our study as a means of analysis of the whole: the substratum unit; and the unit of integrity analysis and modality. Results. As a starting material, we presented a brief analysis of the historical conditions that contributed to the emergence of similar ideological and conceptual views in L. S. Vygotsky, a psychologist, and N. Hartmann, a philosopher, also did not know each other. We examined the fundamental intersection points of: 1) L. S. Vygotsky who is called a Spinozist, and N. Hartmann who gravitated towards Platonism. With similarities of Spinoza and Plato substantial and modal aspects of being, and their identical attitude towards cognition and cognizability; 2) This was reflected in the projects of L. S. Vygotsky and N. Hartman, in particular, in acts of cognition, as “the unity of affect and intellect”, the unity of the cognizer and the cognized, and the definition of the “top” role of the individual; and 3) Both scientists singled out and applied a common method for studying integrity: L. S. Vygotsky - “a unit of analysis of the whole”, N. Hartman - modality. The last conclusion is promising for psychology and allows it to be developed on the basis of a universal ontology. The prospects are as follows: we have shown that the concepts of “units of integrity” and “units of analysis of the whole” are not interchangeable, but in all psychological approaches and concepts, except for the cultural-historical one, the boundaries of these concepts were not given importance and their normative base was blurred. We compared the characteristics of the “unit of analysis of the whole” (L. S. Vygotsky) with the characteristics of modality by N. Hartmann. We determined that these characteristics are similar and made an assumption about the possibility in cultural-historical psychology that “units of analysis of the whole” can be understood as a modality, and not a unit of the whole. This has several advantages: 1) a clearer definition of the boundaries of cultural-historical psychology and its qualitative separation from all other psychologies without the possibility of mixing and substitution; 2) the possibility of analyzing cultural-historical psychology on the basis of a universal ontology, the discovery of new "units of analysis of the whole" (modalities), building links between them; 3) the development of a new psychology - modal psychology, on the basis of cultural-historical psychology; 4) affirmation of the ontological completeness of cultural-historical psychology and embedding psychology on a par with other sciences, which allows building normatively transdisciplinary ties.