Normative concepts form a fundamental category-like, say, temporal or logical concepts. We should not expect to explain time, or logic, in non-temporal or non-logical terms. Similarly, normative truths are of a distinctive kind, which we should not expect to be like ordinary, empirical truths. Nor should we expect our knowledge of such truths, if we have ay, to be like our knowledge of the world around us.
Derek Parfit, ‘Reasons and Motivation’, Proceedings of the Aristotelian Society, supp. vol. 71 (1997), p. 121