What constitutes the meaning of a linguistic expression: the mental states of the language user or external factors? Locke appears to assume the simple thesis that words primarily signify the ideas in the mind of the speaker and thereby to commit himself to an untenable mentalism. The author argues against this widely-held view by providing a comprehensive historical and systematic case that Locke is better described as a social externalist, i.e. someone for whom the linguistic community plays an essential role in fixing meaning.