Darius and Karin Viet #fundie answersingenesis.org
We agree that presuppositional apologetics is the ultimate biblical approach to apologetics. The common accusation that the presuppositionalist uses circular reasoning is actually true. In fact, everyone uses some degree of circular reasoning when defending his ultimate standard (though not everyone realizes this fact). Yet if used properly, this use of circular reasoning is not arbitrary and, therefore, not fallacious.
Contrary to what your non-Christian friend said, circular reasoning is surprisingly a valid argument. The conclusion does follow from the premises. Circular reasoning is a logical fallacy only when it is arbitrary, proving nothing beyond what it assumes.
[Emphasis added]