The Nyāya-sūtra, which is supposed to be the earliest work extant on Nyāya philosophy, treats of four distinct subjects, viz., (1) the art of debate (tarka), (2) the means of valid knowledge (pramāṇa), (3) the doctrine of syllogism (avayava), and (4) the examination of contemporaneous philosophical doctrines (anya-mata-parīkṣā). The first subject, ample references to which are met with in the old Brahmanic, Buddhistic, and Jaina works, seems to have been first handled by a sage named Gotama or Gautama, who is reputed to have flourished in Mithilā (North Behar) about 550 b.c. The second subject, which is also referred to in old books, was associated with the art of debate at a very early stage. These two subjects, combined together, constitute the Tarka - śāstra (the philosophy of reasoning), popularly known as Gautamī vidyā (the Gotamide learning).