<p>This article examines Udayana’s defense of the existence and persistence of the self in the <i>Ātmatattvaviveka</i>. It argues that Udayana’s contribution should be understood as a logical defense in a broad Nyāya sense: an argumentative strategy that combines inference (<i>anumāna</i>), analysis of perception (<i>pratyakṣa</i>), dialectical refutation of rival views, and attention to the conditions of memory, <i>karma</i>, and liberation. The paper first clarifies the Nyāya background and then reconstructs Udayana’s polemical targets, especially Buddhist momentariness, Yogācāra idealism, materialist reductionism, and the objection from non-apprehension. It distinguishes Udayana’s own appeal to indeterminate perception from both earlier inferential accounts of the self and later Naiyāyika theories of internal mental perception. Two central arguments, from memory and from karmic continuity, are reconstructed in formal terms and in a Nyāya five-member format. The article concludes that Udayana’s treatment of the self is not merely a doctrinal defense of <i>ātman</i>, but a sophisticated contribution to global philosophy of religion and to the Logic and Religion project, because it shows how religiously significant metaphysical claims can be assessed through rigorous methods of reasoning.</p>