One of the six orthodox Hindu schools (darshanas). The Nyaya (or, in its alliance with Hindu metaphysics of nature, the Nyaya-Vaisheshika) school concentrates upon our knowledge of reality. Perception, inference, analogy, and testimony are distinguished as the four ways of knowing things. Nyaya generally tends towards a realist metaphysics, and anticipates many later western concerns, for instance with the problem of distinguishing true from misleading perceptions, with induction, and the nature of knowledge via testimony. Nyaya metaphysicians fought running battles with Buddhist philosophers over the doctrine of the reality of the self (See Udayana). The earliest text of the school is the Nyaya-sutra of Gautama, which was the subject of commentary in the succeeding period. Around 1200 the school of Navya-nyaya or new logic was formed, mainly through the work of the logician Gangesa, and the school changed direction from epistemological to more formal concerns.