 Restricting attention to truth equivalence, \({\sim}\) is definable from the operators \(\neg\) and \(\neg{\sim}\). For, \({\sim}\phi\) is true in \(M\) iff \(\neg(\neg{\sim}\phi)\) is true in \(M\). Instead of \({\sim}\), the operator \(\neg{\sim}\) may then be considered. Hintikka points out that the propositional part of \(\mathbf{EIFL}\) (formulated in terms of the operators \(\vee , \wedge , \neg\) and \(\neg{\sim})\) is a Boolean algebra with an operator in the sense of Jónsson &amp; Tarski (1951). The additional operator \(\neg{\sim}\) is a closure operator. 