A non-rigid expression varies its reference from a fixed context across circumstances of evaluation, whereas an indexical expression varies its reference across contexts of utterance

Williamson (2000b) defends Fitch’s reasoning against Kvanvig’s charge. He suggests that Kvanvig has no grounds for thinking that Fitch’s conjunction \(p \wedge \neg \exists x\exists t(Kxpt)\) does not designate rigidly. The reason Williamson gives is this. An expression is non-rigid if, when uttered in a fixed context, it varies its reference with the circumstances under which it is evaluated. But Kvanvig gives no convincing reason for thinking that Fitch’s conjunction, as uttered in a fixed con