If we believe that there are some irreducibly normative truths, we might be believing what we ought to believe. If there are such truths, one of these truths would be that we ought to believe that there are such truths. If instead we believe that there are no such truths, we could not be believing what we ought to believe. If there were no such truths, there would be nothing that we ought to believe.

Derek Parfit, On What Matters, vol. 2, Oxford, 2011, p. 619