The problem of the criterion is a problem in epistemology that creates epistemic circularity via the following loop:
If you know something to be true, then it must satisfy some criterion of truth that you know
If you know the criterion of truth to be true, then it must satisfy the criterion of truth
The implication of the problem of the criterion is that knowledge cannot be well grounded with more knowledge or truth, and instead must be grounded in something else. Common approaches are to ground knowledge in self-consistency, in ungrounded assumptions, or in purpose.