According to some views, natural language suffers from underdeterminacy, but thought doesn’t. According to the underdeterminacy claim, sentence types underdetermine the truth-conditions of sentence tokens. In particular, the semantics of a predicate type seems to underdetermine the satisfaction conditions of its tokens. By contrast, mental representation-types are supposed to determine the truth-conditions of its tokens. In this paper I critically examine these mixed views. First, I argue that the arguments supporting the indispensability of including in one’s theory mental representations that (...) are free of the underdeterminacy exhibited by natural language are not sound. As a result, the possibility that mental representation-types are as underdetermined as natural language sentence-types has not been ruled out. Second, I argue that Carston’s ad hoc concept-types are as underdetermined as word-types. I finish by arguing that mental representations are also underdetermined in a second sense—mental representation-tokens only determine a partial function from possible worlds to truth-values. (shrink)