This paper examines the role of doubt in the classification of inferential reasons in Dharmottara’s Nyāyabinduṭīkā, and in the works of his Nyāya rivals Keśavamiśra and Jayantabhaṭṭa. As deeply systematic thinkers, larger themes in these philosophers’ metaphysics directly determine the more local issues in epistemology and inferential theory, helping to explain, and show the significance of, the differences in their accounts of inferential reasons. For Keśava, an ontology which includes real universals allows for an externalist, reliabilist epistemology, on which the (...) conditions for a genuine inferential reason can be stated in an absolute manner. For Dharmottara, on the other hand, an ontology in which mentally constructed universals are imposed upon real particulars leads to greater dependency on the cognising agent, such that the triple conditions for the inferential reason are formulated in a cogniser-relative way. These differences lead to a more restrictive list of acceptable reasons on Dharmottara’s part, which excludes cases involving doubt. (shrink)