The doctrine of two truths distinguishes conventional or concealing truth (samvriti-satya) from ultimate truth (paramartha-satya). Its purpose is to explain how Buddhist teaching can deny the ultimate existence of persons and things while continuing to speak of them, act, teach, and prescribe a path. Nagarjuna states the requirement flatly in the Mulamadhyamakakarika: those who do not understand the distinction between the two truths do not understand the Buddha's teaching; without relying on the conventional the ultimate cannot be taught, and without realising the ultimate nirvana is not attained.
Conventional truth is the world as ordinarily described and agreed upon: tables, persons, causes and effects, the practice of the path, the difference between a wholesome act and an unwholesome one. Ultimate truth is emptiness, that nothing possesses inherent, independent existence (svabhava). The two are not two realms or two sets of objects; the ultimate truth is the truth about the very things that conventional truth describes.
The doctrine's characteristic danger is nihilism, and Madhyamaka literature is preoccupied with warding it off. Emptiness is not non-existence: things exist dependently, and to say they lack inherent existence is not to say they lack existence. Nagarjuna presses the point further and argues that emptiness is what makes causation and change possible in the first place, since whatever had a fixed inherent nature could not change or be affected. He also warns that emptiness wrongly grasped destroys the unwise, as a snake seized by the wrong end turns and bites, and insists that emptiness is itself empty and must not be converted into a new absolute.
Within Madhyamaka, the interpretive division that Tibetan scholasticism formalised concerns how the ultimate is to be established. The Prasangika position, dominant in Tibetan Madhyamaka, defended by Chandrakirti and given its most influential form by Tsongkhapa, holds that the Madhyamika should advance no positive thesis of his own but only draw out the unwanted consequences of an opponent's position, since any positive assertion would covertly reintroduce inherent existence. The Svatantrika position, associated with Bhavaviveka, holds that autonomous formal arguments may properly be used, and that a conventionally valid establishment of the thesis is needed if the debate is to have any content at all. Whether this was a real division among Indian authors or is substantially a Tibetan doxographical construction is debated by modern scholars. Other traditions deploy the pair for their own ends: Yogacara, Chan, and Tiantai, which adds a third truth of the mean to the two.