hetu

Hetu is a Sanskrit term meaning cause, reason, or evidence. In Indian philosophy and logic, hetu denotes the reason or mark used in inference (anumāna) to establish a proposition about a subject. The concept is central to theories of knowledge in schools such as Nyaya, Vaisheshika, and certain Buddhist logicians like Dignaga and Dharmakirti, where inference is analyzed as a formal argumentative sequence.

In traditional five-member inference, a proposition (pratijna) is supported by a hetu (the reason), illustrated by an udaharana (example), linked to the particular case by upanaya (application), and concluded in nigamana (the result). The hetu is expected to satisfy criteria that make the inference reliable: it should co-occur with the predicate (the sadhya) across all relevant instances (anvaya) and should not occur in instances lacking the predicate (vyatireka). In other words, the hetu should be universally present where the predicate belongs and absent where it does not. It should provide a stable, non-causal or non-arbitrary connection to the sadhya, rather than a merely accidental association. The hetu is typically expressed through a linguistic indicator (hetvabhasa) that signals the inferential link.

A classic example is the inference that a fire is present when there is smoke: smoke is the hetu for fire. Observing smoke in a kitchen (the example) supports the claim that there is fire there (the nigamana). Hetu remains a foundational concept in discussions of evidence, reasoning, and epistemology within Indian philosophical traditions, and it is discussed in relation to how conclusions are justified from observed marks or signs.