Münchhausen trilemma
In epistemology, the Münchhausen trilemma is a thought experiment intended to demonstrate the theoretical impossibility of proving any truth, even in the fields of logic and mathematics, without appealing to accepted assumptions. If it is asked how any given proposition is known to be true, proof may be provided. The Münchhausen trilemma is that there are only three ways of completing a proof
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Source: List of philosophical problems (CC BY-SA 4.0)