What criterion for knowledge?

Modern skeptics try to respond to the ancient ones on an age old problem

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There is a problem in epistemology—the branch of philosophy that deals with the nature of knowledge—that has been raised more than two millennia ago and just doesn’t seem to go away. Here is how the problem, in a nutshell, is rendered by Andrew Cling in his contribution to the edited collection Skepticism—From Antiquity to Present: To know a proposition, we must first know a criterion of truth. To know a criterion of truth, we must first know a proposition. Therefore we cannot know any proposition or any criterion of truth.

To put it differently, the so-called problem of the criterion comes about because (i) whatever answer we give to the question “what do we know about X?” presupposes an answer to the underlying question “how do we know about X?” But (ii) we cannot answer the second question without answering the first one. Which means we can’t really answer either. Ergo, (iii) we don’t know crap, unless we are willing to (iv) engage in circular reasoning in which a proposition is justified by a second proposition, which is then justified by a third one, and so on, until we encounter a proposition that can only be justified by a previous one; or (v) we are okay with an infinite series of justifications, in which the first proposition is justified by a second one, which is justified by a third one, and so on and so forth, forever.