The term nyaya means syllogistic reasoning. As an orthodox system of Indian philosophy, it deals with logical realism. Four means of valid knowledge according to Nyaya System are Perception, Inference, Analogy and Testimony.
The basic text of Nyaya system is Akshapada Gautama’s Nyaya Sutra, written probably in the 4th century BC. Vatsyayana has a commentary on the work in 4th century AD.