requires evaluating the material adequacy of the definition. Some circular definitions will be good analyses, while some will not. Either way, formal correctness Revision theory is a subfield of philosophical logic. It consists of a general theory of definitions, including (but not limited to) circular and interdependent concepts. A circular definition is one in which the concept being defined occurs in the statement defining it—for example, defining a G as being blue and to the left of a G. Revision theory provides formal semantics for defined expressions The liar sentence is not true. On the assumption that the liar is true, one can show that it is false, and on the assumption that it is false, one can show that it is true. This instability is reflected in revision sequences for the liar. The generalization to circular definitions was developed by Gupta, in collaboration with Belnap. Their book, The Revision Theory of Truth, presents an in-depth development of the theory of circular definitions, as well as an overview and critical discussion of philosophical views on truth and the relation between truth and definition. As can be seen in the table, a {\displaystyle a} goes in and out of the extension of G {\displaystyle G} . It never stabilizes. On the other hand, b {\displaystyle b} either stays in or stays out. It is stable, but whether it is stably true or stably false depends on the initial hypothesis. Next, let D 2 {\displaystyle {\mathcal {D}}_{2}} be H x = D f H x ∨ ∼ H x . {\displaystyle Hx=_{Df}Hx\lor \sim Hx.} As shown in the following table, all hypotheses for the ground model of the previous example are revised to the set {a, b} .