This paper argues in favor of a fundamentally new perspective on phonology via modal logic. We show that the class of total Boolean Monadic Recursive Schemes (BMRS), used in computational modeling of phonological processes (Bhaskar et al., 2020; Chandlee Jardine, 2021), is equivalent in expressive power to the well-studied modal 𝜇-calculus. As a corollary of this result, we obtain an alternative proof that order-preserving BMRS transductions capture the class of rational functions, which have been posited as a complexity bound on natural language phonological grammars.