 The two greatest names for classical Indian logic belong to logicians of the Buddhist Yogācāra school, Dignāga (sixth century) and Dharmakīrti (early seventh century). Dignāga laid out all the possible relationships of inclusion and exclusion for the extensions of two terms called the prover or “sign,” hetu, and the probandum, sādhya, the property “to be proved.” Thereby he revealed the underpinnings of the pramāṇa of inference in terms of sets of particulars, which, according to Yogācāra ontology, are the only reals. Dharmakīrti classified inferences based on the ontological nature of the class-inclusion relationship that underpins all inference as a knowledge source. Earlier philosophers, both Buddhist and non-Buddhist, provide examples of everyday reasoning, several of which are abductive in character, informal reasoning to the best explanation, from sight of a swollen river, for example, says Vātsyāyana in his commentary on the inference sūtra (1.1.5) of the Nyāya-sūtra, to the conclusion that it has rained upstream. But there are also instances of inferences comprised of deductive, extrapolative, and sometimes properly inductive reasoning on topics of everyday life as well as philosophy in numerous pre-Dignāga texts of several schools. It is not true, as is sometimes claimed, that no one before Dignāga had the notion of an inference-underpinning “pervasion,” vyāpti, of a prover property by a property to be proved. Dignāga does however get the credit for the earliest systematization, which employs three terms, a site or subject of a proposed inference (pakṣa, the mountain in the stock example of an inference from sight of smoke on a mountain to knowledge of fire on the mountain), the prover or prover property (hetu, smokiness), and the probandum (sādhya, fieriness).