Drawing on the results of contemporary Russian Buddhist studies, the authors problematize such well-established ideas as the status of "I", understanding of nirvana, the denial of the absolute, etc. It is shown that these concepts are not so unambiguous, they need to be clarified and deepened; they can have a contradictory dialectical character, this is especially true of such Mahayana schools as the Tathagatagarbha and the Vajrayana. The analysis of these problematic concepts and their clarification are necessary for an adequate understanding of both Buddhism in its historical development and the history of philosophy as a whole. The authors reveal the unanimity of views of contemporary Russian Buddhologists on the need to distinguish between the empirical and the possible absolute "Self" in Buddhism, as well as to outline the difference in their ontological statuses. The article also provides a comparison with the concepts of "Self" in Western philosophical schools of thought, especially in the criticism of Immanuel Kant. The researchers note both the commonality of transcendental beliefs in Buddhism and Kantianism, and the difference in attitude to metaphysics, understanding of the spiritual and ethical. They consider apophatism in definition of nirvana and the transcendent and immanent nature of Buddhism in comparison with other teachings, primarily Christianity. The revealed problematicity of determining the fundamental principles and concepts of Mahayana requires further in-depth study of the history of the development of the philosophy of Buddhism. © 2021 New Reaearch of Tuva. All rights reserved.