Circular Reasoning

There is a larger sense in which circularity, as Pontificator suggests, cannot be avoided in arguing for the ultimate criterion of a system. This is what Aristotle and St. Thomas Aquinas meant by saying that first principles are indemonstrable. Why should one be logical? Because it would be illogical not to be! Why should one believe God's Word? Because it is the Word of God, of course! Every system is based on presuppositions that control its epistemology, argument, and use of evidence; therefore ultimate circularity is philosophically inescapable. But this does not mean that circularity is permissible in other (penultimate) sorts of arguments. "The Bible is inspired because the Bible says its inspired" is a circular argument whose circularity is not justified. It lacks cogency. A document's self-attestation is insufficient warrant for accepting its claims. The argument can gain cogency only by enlarging its circle to include also the attestation of the Church and data of sacred and secular history. By contrast, "The Bible means what the Church says it means" is not circular in this way, since the Church's interpretation is not closed off from history, but empirically testable for fidelity and coherence both against Scripture and the other traditions of the Church.

See Link above for full article, it's not too long either.