Three Truths 

A doctrine developed by Chih-i as a result of dissatisfaction with the essentially negative metaphysical analysis of the Madhyamaka teachings. The problem, as he saw it, was that the ‘Two Truths’ of Madhyamaka presented the Ultimate Truth as simple negation: it said what things were not, without making any positive statement of what they were. Thus, Chih-i proposed the Three Truths: the truths of emptiness (śūnyatā), provisionality, and the middle. The first broke down all illusions about things, denying their permanence and the existence of any essence within them. The second affirmed their existence as impermanent objects that arose, abided, decayed, and ceased according to the laws of cause and effect. Thus, these first two truths corresponded to the Ultimate (paramārtha-satya) and Conventional (saṃvṛti-satya) Truths of Madhyamaka. The third truth, that of the middle, synthesized these into a positive statement about the nature of reality. The impermanence (anitya) and interdependence of all phenomena was the ultimate truth about them. In this way, Chih-i denied that emptiness and provisionality were two different and unrelated aspects of things, or that emptiness negated provisionality, but stated that the contingency of things was in itself the ultimate truth about them. T'ien-t'ai doctrine, as developed by Chih-i, thus embodies a view of an immanent transcendent. That is to say, it does not look for a pure, undefiled realm that is above and beyond the present, defiled world. Rather, by positing the threefold truth, it affirms that the absolute abides in and through the contingent, not outside or beyond it. This found expression in one of the characteristic doctrines of the school—that of the non-obstruction of phenomena with the absolute (Chinese, li shih wu ai).

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)