Commonly we demonstrate the truth of a proposition by providing proof. But our doubter might then turn his skepticism on the proof in its turn. It seems there are only three ways to reach the end of the business:

• by a circular argument, in which the proof of a proposition presupposes its truth
• by a regressive argument, in which each proof requires a further proof, and so on forever
• by a dogmatic argument, in which precepts are asserted rather than defended

This is called the Münchhausen trilemma after Baron Münchhausen, who tried to lift himself and his horse out of a mire by pulling on his own hair. Any attempt to justify knowledge must start from a position of ignorance. Without firm ground to stand on, it seems, there’s no way to “bootstrap” ourselves into confident assertions.