Conventional truth (-sacca) is "commonly accepted truth," the consensus reality, true at their own level like Newtonian physics. In ancient India it was called samvrti-satya (Sanskrit) or sammuti-sacca (Pali).
The Buddha expressed and explained the Dharma using the language of both kinds of truth. The sutras are characterized by the use of ordinary language, whereas the "higher teachings" (Abhidharma) characteristically employ ultimate language, true at their own level as in quantum physics where Newtonian truths simply do not hold up.
It does so because it is a super-accurate mode of expression that accords with insight into reality. There are things as they seem in ordinary experience and things as they really are. It is sometimes very important to distinguish.
Our experience of life is as consciousness embodying or incarnating into this existence. Self is real and we go about as owners of (1) body, (2) feelings, (3) perceptions, (4) will, and (5) consciousness. This is all well and good, and this is how we speak, but it is not ultimately true.
In an ultimate sense, any existence is merely the constant arising and falling away of these Five Aggregates. So life can be seen as a functionally-integrated process of physical (body) and psychological (mind/heart) phenomena. Within it and beyond it no real ego-entity or abiding substance is found.
This good, this is liberating, but it is very hard to see. What we consider existential problems are not really problems, yet we often fail to see the danger of this continued wandering on through the illusion.
It is all well and good to study the Dharma and parrot these things, but that in no way should be confused with grasping them and being liberated by the truth. The conventional will be true for us until we break through to the real, which can only be accomplished by practice of what is learned.
- How dry and boring would it have been if the Buddha, having awakened, suddenly began to speak exclusively in ultimate terms? Like a professor-scientist, a particle physicist, speaking only of quarks and neutrinos, points and waves, abstractions and things meaningless to the average person, the Dharma would not still be in the world. What did the Buddha do instead? Instead he used ordinary language, parables, fables (jatakas), stories, humor, histories, mythologies...and ultimate language. He directly reached people, and we therefore have the Dharma. He did not teach to say who he was but to bring us to a realization of what we are: He brought people to knowledge-and-vision of the truth. There is nothing to believe. So the best place to learn "Buddhism" in a meditation hall not a lecture hall.
Whenever the sutras speak of a person or of the rebirth of a being, these assertions must not be confused with what is valid in an ultimate sense. There are merely conventional ways of speaking (-vacana).
One of the striking characteristics of the Abhidharma, one of the three divisions of the Dharma (as distinct from the Discourse and Discipline), is that it is interested in cutting through conventions. It deals with ultimates. It lays bare realities in the highest sense (paramattha-dhammā).
The sutras or discourses contain many expositions in terms of ultimate language. This happens, for example, when they deal with the Five Aggregates, elements, or sense-bases and their constituents. It also happens when recognizing the Three Characteristics of Existence: the impermanent, disappointing, and impersonal nature of composite things.
The majority of discourses, however, use conventional language. This is appropriate in a practical or ethical context. For it would stretch conventional language to say, for example, that "the aggregates feel feelings," or "consciousness reincarnates," and so on.
The Buddha's statements that use conventional language are also "truth," correct on their own level of exposition. Conventional terms do not contradict the fact that such statements ultimately refer to impermanent, disappointing, and impersonal processes. In fact, this is why those impermanent and impersonal processes are sometimes spelled out.
The Buddha repeatedly mentioned his reservations when using conventional speech, for example in the ninth sutra of the Longer Discourses (DN 9): "These are merely names, expressions, turns of speech, designations in common use in the world, which the Tathāgata (the Buddha) uses without misapprehending them." (See also S. I. 25).
The term paramattha (ultimate truth), in the sense used here, occurs in the first paragraph of the Kathāvatthu, an Abhidharma work (see Guide, p. 62).
But commentarial discussions on these truths (i.e., commentaries to DN 9 and MN 5) have not yet been fully translated. On these see K.N. Jayatilleke's Early Buddhist Theory of Knowledge (London, 1963), pp. 361ff.
In Mahāyana, the Mādhyamika school has given a prominent place to the teaching of the two truths.