The relevance of the study is caused by the need of understanding the cognitive capabilities of human reason within verification of origin of the latter. Such perspective is conditioned by orientation of research on the development of a holistic set in epistemology, which will develop a holistic methodology for knowledge according to most researchers. Appealing to "I" ontology is one of the ways of forming such facility in epistemology. The objective of the study is to identify cognitive bases of "I" ontology through a comparative analysis of their interpretation in the history of epistemology, to establish potential dependence of determining the horizon of human cognitive abilities on his epistemological "I"-concepts through the lens of knowledge problematization mechanism. Methods: the comparative analysis, holistic approach. Results: The author has considered the ontological "I" concepts in the history of epistemology. The paper demonstrates the dependence of nature and limits of knowledge regarding on understanding of "I" ontology as an essential dimension of man. It is noted that ignoring the holistic understanding of "I" ontology leads to a simplification of cognitive activity results. It is stated that the human self-conception is relevant in understanding knowledge problematization and in determining knowledge horizon.