Abstract:  Some linguistic phenomena can occur in uses of language in thought, whereas others only occur in uses of language in communication. I argue that this distinction can be used as a test for whether a linguistic phenomenon can be explained via Grice’s theory of conversational implicature (or any theory similarly based on principles governing conversation). I argue further, on the basis of this test, that conversational implicature cannot be used to explain quantifier domain restriction or apparent substitution failures involving coreferential names, but that it must be used to explain the phenomenon of referential uses of definite descriptions. I conclude with a brief discussion of the relevance of this point to the semantics/pragmatics distinction.