Proposition 78: Reflexivity of equivalence (inference rule)

We may (from no premises) infer that the equivalence \phi \Leftrightarrow \phi holds: \cfrac{}{\vdash (\phi \Leftrightarrow \phi)}.

Proof: By the identity rule, we know that \phi entails \phi . Using this as both premises in biconditional introduction, we infer that \phi \Leftrightarrow \phi holds. \square