In the Thorough Investigation of Truth (Tattvasamīkṣā), a commentary on Maṇḍana Miśra’s The Demonstration of Brahman (Brahmasiddhi), Vācaspati argues for the truth of the Upaniṣadic claim of non-duality between self and reality, against an interlocutor who would interpret the Upaniṣads in a figurative manner. At stake is the role of language in relationship to other sources of knowledge. The opponent objects to the conflict between sense perception and the putatively ordinary meaning of the Upaniṣads, since this would entail that sense perception—which delivers knowledge of a world apart from the self—is flawed. They argue that, when such conflicts arise, perception should prevail, and a figurative meaning should be preferred. Vācaspati objects on a number of grounds, first noting that there must be a relationship between a primary and secondary meaning (one cannot simply leap to a secondary meaning when it is convenient). Then he appeals to the principle already described in detail—when it is not possible to interpret the ordinary meaning in a sensible manner, this is the impetus for secondary meaning. Conflict with other knowledge sources is not the catalyst for figurative interpretation (Vācaspatimiśra, VT: 29–32).