This chapter focuses on John Locke's theory of language, and considers more generally what one might expect a philosophical theory of language to achieve. It examines the merits of Locke's approach to the nature of language and thought. Locke's interest in language seems to focus first and foremost on its expressive character rather than on its semantic relations and properties. The chapter analyzes what Locke believes to be the basic function of language. The privacy of ideas may appear to create a serious difficulty for Locke. Locke himself insists on this privacy, saying that ideas are "invisible, and hidden from others". Locke's theory of abstraction is a theory which he advances in order to explain how the general terms in a language play their distinctive roles. In the Essay, Locke offers brief but quite different account of the linguistic function of what he calls "particles".