text-book of this system is the Nyāya-sūtra of Gotama. The importance here attached to logic appears from the very first aphorism, which enumerates sixteen logical notions with the remark that salvation depends on a correct knowledge of their nature.
Neither the Vaiçeshika nor the Nyāya-sūtras originally accepted the existence of God; and though both schools later became theistic, they never went so far as to assume a creator of matter. Their theology is first found developed in Udayanāchārya's Kusumānjali, which was written about 1200 A.D., and in works which deal with the two systems conjointly. Here God is regarded as a "special" soul, which differs from all other individual eternal souls by exemption from all qualities connected with transmigration, and by the possession of the power and knowledge qualifying him to be a regulator of the universe.
Of the eclectic movement combining Sānkhya, Yoga, and Vedānta doctrines, the oldest literary representative is the Çvetāçvatara Upanishad. More famous is the Bhagavadgītā, in which the Supreme Being incarnate as Kṛishṇa expounds to Arjuna his doctrines in this sense. The burden of his teaching is that the zealous performance of his duty is a man's most important task, to whatever caste he may belong. The beauty and the power of the language in which this doctrine is inculcated, is unsurpassed in any other work of Indian literature.
By the side of the orthodox systems and the two non-Brahmanical religions, flourished the lokāyata ("directed to the world of sense"), or materialistic school, usually called that of the Chārvākas from the name of the founder of the doctrine. It was regarded as peculiarly heretical, for it not only rejected the authority of the