 In this theory, a circular definition imparts to the defined term a meaning that is hypothetical in character; the semantic value of the defined term is a rule of revision, not as with non-circular definitions, a rule of application. Consider (18) again. Like any definition, (18) fixes the interpretation of the definiendum \(if\) the interpretations of the non-logical constants in the definiens are given. The problem with (18) is that the defined term \(G\) occurs in the definiens. But suppose that we arbitrarily assign to \(G\) an interpretation—say we let it be the set \(U\) of all objects in the universe of discourse (i.e., we suppose that \(U\) is the set of objects that satisfy \(G)\). Then it is easy to see that the definiens is true precisely of Socrates and Plato. The definition thus dictates that, under our hypothesis, the interpretation of \(G\) should be the set \(\{ \text{Socrates}, \text{Plato}\}\). A similar calculation can be carried out for any hypothesis about the interpretation of \(G\). For example, if the hypothesis is \(\{\text{Xenocrates}\}\), the definition yields the result \(\{\text{Socrates}, \text{Aristotle}\}\). In short, even though (18) does not fix sharply what objects fall under \(G\), it does yield a rule or function that, when given a hypothetical interpretation as an input, yields another one as an output. The fundamental idea of the revision theory is to view this rule as a revision rule: the output interpretation is better than the input one (or it is at least as good; this qualification will be taken as read). The semantic value that the definition confers on the defined term is not an extension—a demarcation of the universe of discourse into objects that fall under the defined term, and those that do not. The semantic value is a revision rule.