In An Essay Concerning Human Understanding Locke makes an extravagant claim: Morality is as capable of demonstration as Mathematics. In the sixth Conversation between the seventeenth-century philosopher John Locke and the student of language Terence Moore, Moore points out that Locke's own arguments on the nature of language demonstrate that morality in a strong sense is not demonstrable. The Conversation then turns to Locke's real concern – ways in which words used in moral judgements might be made less ‘uncertain, vague, ambivalent’.