 The lesson is that we may not substitute unrestrictedly into modal contexts. Substitution into modal contexts, we might say, is permitted only if the substituting terms are rigid designators. In the case of Fitch’s result, our terms are sentences. The knowability principle, \(\forall p(p \rightarrow \Diamond Kp)\), apparently allows us to substitute any sentence whatsoever for \(p\). But notice that our quantifier has wide scope relative to \(\Diamond\). We would expect that the lessons of quantified modal logic carry over to quantified propositional modal logic. If so, then we may not substitute for \(p\) any statement that does not designate rigidly.