Why ought I to do what I know that I ought to do?
[…] I might ask [this] if I knew that I ought to do something, but I didn’t know, or had forgotten, what made this true. Such cases raise no puzzle. Suppose next that, though I know both that and why I ought to do something, I ask why I ought to do this thing. The only puzzle here would be why I asked this question. When we know why something is true, we don’t need to ask why this thing is true.
Derek Parfit, On What Matters, Oxford, forthcoming
We tell someone that such and such a thing is what morality requires, and he replies that he agrees with us but does not see why he should do what morality requires. What could we say in reply? The individual could have reasons of prudence to do the same thing that morality requires, but, if he asks that question, it is probable that he does not have those reasons or that they are not enough for him. But, if they are not reasons of prudence, what other kinds of reasons is he looking for? What is the meaning of ‘should’ in the question ‘why should I be moral?’ The only possible answer is that the reasons in question must be moral ones and that the duty alluded to by the expression ‘should’ must be amoral duty, since our practical reasoning does not admit reasons and duties of a higher order. But the person who asks these questions will not, of course, be satisfied with an answer which presupposes what he is doubting. What is he in fact asking? The very question seems to involve a contradiction, since once adequately articulated it reads: What moral reason do I have to do what morality prescribes, which is not a reason which is derived from morality itself? This is like asking who is the lucky woman who is the wife of the richest bachelor on earth, and being distressed that we do not get an answer.
Carlos Santiago Nino, The Ethics of Human Rights, Oxford, 1991, pp. 81-82