A new book studies ancient texts and ideas to identify six systems of Indian philosophy

The five schools of philosophy that preceded Shankara’s systemic exposition of Vedantic metaphysics were the Nyaya, Vaisheshika, Sankhya, Yoga, and Purva Mimamsha. All these were essentially guided by two fundamental tenets, investigation or mimamsha, and reflection or vichara – about the ultimate nature of the world, and the consequential purposes of life. They overlapped in their concepts and reasoning in some respects, but their differences were equally marked, and in this sense, provide definitive proof of the eclectic milieu of those times, and the independence and robustness of thought they nurtured.

The Nyaya Sutra dates back to the third century BCE and is attributed to the sage Gautama. This school’s principal preoccupation is with logic and dialectics, analysis and reasoning. To this end, the Nyaya relied primarily on four sources of knowledge: perception (pratyaksha), inference (anumana), analogy (upamana) and verbal testimony (shabda). Such tools were essential, the Nyaya Sutra stressed, to establish whether that which is posited exists or not. In other words, the importance of Nyaya lies in the fact that it set out the analytical framework for enquiry, and refused to accept anything only on face value or assertion.

The Vaisheshika school of the sage Kanada (third century BCE) relied...