Pakṣilasvāmin Vātsyāyana (c. 450 CE) is the author of the Commentary on Nyāya (Nyāya-bhāṣya), the first full commentary on the Nyāya-sūtra of Gautama (c. 150 CE), which is itself the foundational text of the school of philosophy called “Nyāya.” The Nyāya tradition is home to a number of leading voices within the classical Indian philosophical scene and is celebrated in later doxographies as one of the six “orthodox” systems of Hindu thought. Given the way that sūtra texts and their first (...) commentaries are profoundly intertwined, Vātsyāyana’s work provides a formative vision of Nyāya’s self-conception as a philosophical system as well as interpretive strategies, central lines of argumentation, and determinations of importance that later Naiyāyikas (Nyāya philosophers) often take to be as much as a given as the original text itself. Along with its sustained defense of metaphysical realism across various fronts, Nyāya is best known for its achievements in epistemology and logic, and as indicated by the title of this chapter, the lens through which we will explore Vātsyāyana’s thought is his theory of knowledge. We will give special attention to his account of the nature and importance of cognition as a guide to action and will illustrate the way in which this theme informs a number of apparently distinct elements of his project including his realism, his account of epistemic entitlement, and his notion of philosophy’s contribution to living well. (shrink)