All knowledge is circularly justified
Many philosophers have tried to find the foundations of our knowledge, but why do we think there are any? The framing of foundations implies a separate bottom layer of knowledge from which everything is built up. And while this is undoubtedly a useful model in many contexts, why should we believe in this as the complete and literal truth as opposed to merely a simplification?
1) If we dig deep enough into any of our truth claims, we’ll eventually reach a point at which they are justified by intuition
2) The reliability of intuition or various intuitions is not something that is merely taken as basic or for granted, but can instead be justified somewhat by arguments from experience and evolutionary arguments.
3) However both empirical verification and evolutionary arguments themselves both rely on assumptions that are justified by intuition
This is circular, but is this necessarily a problem? If your choice is a circular justification or eventually hitting a level with no justification, then the circular justification suddenly starts looking pretty attractive.
Is this important? It seems to depend on context. For applied rationality, not so much. But perhaps the more philosophical areas of the rationalist project would look quite different if they were built upon a circular epistemology.