Intellectual setting
Dignāga wrote as a critic of the Nyāya school's realist epistemology and of earlier Buddhist Abhidharma logic, and his innovations provoked direct responses from Nyāya logicians, setting the terms for centuries of debate over perception, inference, and universals in Indian philosophy.
Key ideas
Two valid means of knowledge
Dignāga argued that only perception (pratyakṣa), which is free of conceptual construction and grasps unique particulars directly, and inference (anumāna), which operates through conceptually constructed general marks, count as valid means of knowledge, rejecting other pramāṇas accepted by rival schools as reducible to these two.
The wheel of reasons (hetucakra)
Dignāga devised a systematic ninefold classification of possible relations between a logical reason and what it is meant to prove, showing which combinations yield valid inferences and which produce fallacies, laying the groundwork for later Buddhist formal logic.
Legacy
Dignāga's epistemology was systematized and extended by his most important successor, Dharmakīrti, and together their work became the foundation of Buddhist logic studied throughout Tibet, Central Asia, and East Asia; his nominalism about universals remains a reference point in comparative philosophy of language.