Dharmakīrti inherited Dignāga's framework and transformed it. His Pramāṇavārttika (Commentary on Valid Cognition) is not a humble gloss but a creative renovation, Dharmakīrti tightens the definitions, answers objections, and introduces a concept that would anchor the whole system: arthakriyākāritva, causal efficacy, or the capacity to perform a function.
For Dharmakīrti, a real thing is precisely one that can make something happen. The unique particular (svalakṣaṇa) is real because it has determinate causal powers, it can heat, illuminate, feed, wound. Universals and conceptual constructs lack this capacity: "cowness" does not give you milk; only this cow, right here, does. This causal criterion is both a metaphysical claim and an epistemological one: perception is the pramāṇa that tracks real things because it is itself caused by those things, receiving their causal imprint directly.
Dharmakīrti also deepens the apoha theory. Where Dignāga's version was largely semantic, words exclude rather than include, Dharmakīrti gives it a cognitive and pragmatic dimension. When you hear the word "fire" and look for something to warm yourself, what guides your action is not a reference to a universal but the cognition of something different from everything that would fail to warm you. The exclusion is not just a logical operation; it tracks the causal structure of the world as it bears on your goals.
A word indicates an object merely through the exclusion of other objects (anyapoha, -vyavṛtti). For example, the word "cow" simply means that the object is not a non-cow. As such, a word cannot denote anything real, whether it be an individual, a universal, or any other thing. The apprehension of an object by means of the exclusion of other objects is nothing but an inference.
— Dignāga, Pramāṇa-samuccaya, summarized by Hattori Masaaki
Dharmakīrti's epistemology also has a critical edge. The Buddhist logical tradition accepts scriptural tradition only if it accords with pratyakṣa and anumāna. This view is in line with the Buddha's injunction in the Kalama Sutta not to accept anything on mere tradition or scripture. Dharmakīrti applied this ruthlessly: even core Buddhist doctrines had to be defensible through perception and inference. Religious authority is not a third source of knowledge; it either confirms what perception and inference already establish, or it is not knowledge at all.
This tradition represents among the most sophisticated pre-modern developments in formal logic, philosophy of language, and epistemology anywhere in the world. Its influence extended to Tibet (through the Gelug tradition's intensive study of Dharmakīrti's works), to China, and, through modern scholarship, into contemporary analytic philosophy of mind and language.