Invoking metaphysical naturalness is “perhaps the most popular proposed solution” to the problem of grue (Hedden in Can J Philos 45:716–743, 2016). Accordingly, Bradley (Mind 129:179–203, 2020) develops a “Lewisian” method for constraining priors based on the syntactic simplicity of descriptions of possible worlds in a language whose predicates correspond to natural properties. The Lewisian method therefore requires a solution to the arguably unsolved problem of measuring syntactic simplicity. But this paper argues that _given_ a solution to this problem, there (...) is a better alternative to the Lewisian method: a novel “Neo-Carnapian” method for constraining priors based on the indistinguishability of experiences. (shrink)