Nyāya Sūtras 42

And regarding this object of hypothetical reasoning—

Ascertainment is the determination of an object by means of thesis and counter-thesis after having considered [it].

Proof is establishment; refutation is objection. Those two, establishment and objection, resting upon thesis and counter-thesis, intertwined and proceeding continuously, are called 'thesis and counter-thesis'. Of the two, the cessation of one and the remaining of the other is inevitable; of that which remains, the determination of the object is ascertainment. [Objection:] 'This determination of an object by both thesis and counter-thesis is impossible.' For one person establishes by a logical reason an asserted object and removes [the objection] refuted by the second person; by the second, the logical reason for the establishment is refuted and the logical reason for his own refutation is raised, and that [first one] ceases.

Upon its cessation, that which remains—by that the determination of the object is ascertainment. [Response:] He says that the determination of the object is by both. By what reasoning? The possibility of the one and the impossibility of the second: these two, possibility and impossibility, together eliminate the consideration. If both were possible or both were impossible, the consideration would not cease. 'Having considered' means having made a consideration. This consideration, illuminating thesis and counter-thesis, sets reasoning in motion; therefore it is included. And this should be understood [only] in the case of two contradictory [properties] residing in a single locus. But where contradictory properties relating to a common class of loci are possible through logical reasons, there is combination, because through logical reasons the object is proved to be so. For example, in the definitional statement 'a substance is that which possesses action', that substance for which connection with action is possible through a logical reason is possessing action, and that for which it is not possible is actionless. And in the case of two contradictory properties residing in a single locus that do not exist simultaneously, there is temporal differentiation. For example, that very substance, when joined with action, possesses action, but when its action has not arisen or has ceased, it is actionless. And this is not a restrictive rule for ascertainment—that ascertainment is the determination of an object only after having considered through thesis and counter-thesis; rather, in perception arisen from the contact of sense organ and object, the determination of an object is ascertainment, but in the realm of critical investigation, ascertainment is the determination of an object by thesis and counter-thesis after having considered; and in philosophical debate and treatise, it is devoid of consideration. ||41