The Nyāya Sūtras is an ancient Indian Sanskrit text composed by Akṣapāda Gautama, also known as Aksapāda, and is the foundational text of the pracina or Old Nyaya school of Hindu philosophy.
The text contains 5 parts divided further into chapter and contains over 500 sutras.
Babaji is reading from Nyāya-darśanam with the Vātsyāyana's bhāsya, which includes the Hindi commentary by Dhundhiraj Shastri.
