When a man speaks to another, it is that he may be understood
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“words, in their primary or immediate signification, stand for nothing but THE IDEAS IN THE MIND OF HIM THAT USES THEM, how imperfectly soever or carelessly those ideas are collected from the things which they are supposed to represent. When a man speaks to another, it is that he may be understood: and the end of speech is, that those sounds, as marks, may make known his ideas to the hearer. That then which words are the marks of are the ideas of the speaker: nor can any one apply them as marks, immediately, to anything else but the ideas that he himself hath:” source