Raghunātha, a fifteenth-century Nyāya scholar, composed a short treatise called Padārthatattvanirūpaṇa, in which he criticized the Vaiśeṣika’s category system accepted at the time and claimed the necessity of radical revisions of the system. Among a number of targets of criticism stands the reductionism of “moment” of kṣaṇa. Traditionally, Vaiśeṣika holds that people refer to a motion of a substance when they delimit time and measure moments. Raghunātha, however, does not think that a motion of a substance helps us measure moments, and postulates the existence of kṣaṇa, which is a momentary entity and irreducible to any of the known categories. In this paper, I survey the history of the controversy between reductionism and anti-reductionism of kṣaṇa in Vaiśeṣika, focusing on Vallabha, Vardhamāna, and Raghunātha, and present the close relation of Ragunātha to Vardhamāna. Based on this historical survey, I further examine the philosophical significance of postulating kṣaṇa as a new category. The most significant point may be that the metaphysical necessity for postulating kṣaṇa is derived from the Vaiśeṣika’s popular theorem of causality that the diversity of causes prohibits the uniformity of results.
