A question asked without expecting an answer, because the answer is already implied, or because what matters is the doubt it exposes.
In Paradise
now who are you, who want to sit in the chair to judge from a thousand miles away with the short sight of a span?
"Who are you?" asks for no answer: the Eagle uses the question to refuse the judge's seat to the one who asks it. Dante's doubt is not undone, but returned to him with the measure of what he cannot reach.