operator a one-place sentential connective; i.e., an expression that may be prefixed to an open or closed sentence to produce, respectively, a new open or closed sentence. Thus ‘it is not the case that’ is a (truth-functional) operator. The most thoroughly investigated operators are the intensional ones; an intensional operator O, when prefixed to an open or closed sentence E, produces an open or closed sentence OE, whose extension is determined not by the extension of E but by some other property of E, which varies with the choice of O. For example, the extension of a closed sentence is its truth-value A, but if the modal operator ‘it is necessary that’ is prefixed to A, the extension of the result depends on whether A’s extension belongs to it necessarily or contingently. This property of A is usually modeled by assigning to A a subset X of a domain of possible worlds W. If X % W then ‘it is necessary that A’ is true, but if X is a proper subset of W, it is false. Another example involves the epistemic operator ‘it is plausible that’. Since a true sentence may be either plausible or implausible, the truth-value of ‘it is plausible that A’ is not fixed by the truth-value of A, but rather by the body of evidence that supports A relative to a thinker in a given context. This may also be modeled in a possible worlds framework, by stipulating, for each world, which worlds, if any, are plausible relative to it. The topic of intensional operators is controversial, and it is even disputable whether standard examples really are operators at the correct level of logical form. For instance, it can be argued that ‘it is necessary that’, upon analysis, turns out to be a universal quantifier over possible worlds, or a predicate of expressions. On the former view, instead of ‘it is necessary that A’ we should write ‘for every possible world w, A(w)’, and, on the latter, ‘A is necessarily true’. See also INTENSIONAL LOGIC, MODAL LOGIC , POSSIBLE WORLD. G.Fo.