  Gupta and Belnap's related suggestion concerning the ‘iff’ in the T-biconditionals is that, rather than being the classical biconditional, this ‘iff’ is the distinctive biconditional used to define a previously undefined concept. In 1993, Gupta and Belnap present the revision theory of truth as a special case of a revision theory of circularly defined concepts. Suppose that L is a language with a unary predicate F and a binary predicate R. Consider a new concept expressed by a predicate G, introduced through a definition like this: