(Sektion 217: Philosophische Untersuchungen von Ludwig Wittgenstein).
217. "How can I follow a rule?"--if this is not a question after the reason (Ursachen), then it is a question after the justification (Rechtfertigung) for my acting the way that I do.
If I have exhausted the justifications, I am at bedrock and my spade turns back at itself (und mein Spaten biegt sich zurück). I am then inclined to say: "Thus I act simply."
(Remind yourself that we sometimes demand explanations not for the sake of their content, but for the sake of the form of the explanation. Our requirement is an architectural one; the explanation is a kind of ornament that does not support.)