All attempts to disprove the reality of moral obligation, to show that it is an illusory notion to which nothing or only something very different really corresponds, fail. But it may be said that all attempts to prove its reality are equally futile. And this is true. You can no more prove that there are duties than that there are beautiful things or true judgements. The truth of some judgements and the existence of some duties are self-evident. And nothing is more certain than what is self-evident, for that does not need or gain by proof and is generally incapable of it. Everything that can be proved is proved from something which is itself evident, ultimately from something self-evident, and by arguments whose validity is self-evident and cannot be proved. If everything had to be proved nothing could be, for the process would be infinite, and infinite in two directions, an infinite regress of premises to be proved and an infinite regress of proofs to be validated. The distinction of right and wrong can no more be deduced from any non-moral conception than that between truth and falsehood from anything non-rational. We see the necessity of both with equal clearness. To fail of making either would be an equally sure mark of insanity, of falling outside the human pale.
If any one ask us, 'Why ought I to do these acts you call my duty?' the only answer is, 'Because they are your duty', and if he does not see this we cannot make him, unless by informing him about matters of fact; if he sees they are duties, he can no more ask why he ought to do them than why he should believe what is true. To answer, 'Because they are the best policy', would not answer the question why he ought to do them.
(E. F. Carritt, The Theory of Morals: An Introduction to Ethical Philosophy [London: Oxford University Press, 1928], 28-9 [italics in original])