Since Frege 1879, the history of semantics identifies the meanings of natural language expressions with the mind external things they denote, be they pedestrian objects (e.g., cows and chairs), less pedestrian objects (e.g. mereological sums), or abstracta (e.g., sets of possible worlds). For the Quinean Realist, a language with such a semantics is fruitful for ontological investigation, insofar as analyzing the denotational meanings of (the constituents of) sentences in that language reveals which objects populate the (external) worldly domain. However, consigning meaning over to truth in this manner comes at a cost. The externalist thesis is only had by sacrificing the explanatory adequacy of our theory of meaning. Three arguments suggest this: first, facts about the rapid human acquisition of natural language suggests that languages are internal to the human mind, as an innate module in cognitive architecture; second, naturalist commitments suggests that there is no sui generis, mind-independent kind ‘word’ to stand in the word-to-world relations posited by the externalist; third, natural languages exhibit a lexical flexibility, as manifest in the distribution of natural language speaker judgments, and this property cannot be easily explained by an externalist semantics. The Realist might respond to these arguments by appealing to the languages used to express out best scientific theories, using those invented languages as ontological guides. Since these scientific languages are constructed with the expressed purpose of perspicuously describing reality, the Realist could contend that expressions in those languages have an externalist semantics. I argue, using examples from evolutionary biology, that scientific languages exhibit lexical flexibility as well, casting doubt on the claim that these languages have meanings that admit to externalist treatment. The Realist then should reject the metaphysical methodology which assumes the externalist assumption that the meaning of a linguistic expression determines its truth-conditions.