A man cannot make his words the signs either of qualities in things, or of conceptions in the mind of another, whereof he has none in his own.
 John Locke, An Essay Concerning Human Understanding (1689). copy citation

Context

“and so in effect to have no signification at all. Words being voluntary signs, they cannot be voluntary signs imposed by him on things he knows not. That would be to make them signs of nothing, sounds without signification. A man cannot make his words the signs either of qualities in things, or of conceptions in the mind of another, whereof he has none in his own. Till he has some ideas of his own, he cannot suppose them to correspond with the conceptions of another man; nor can he use any signs for them: for thus they would be the signs of he knows not what, which is in truth to be the signs of nothing.” source