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