I have been studying "presuppositional apologetics" for the purpose of open air preaching on Universities and it has helped a great deal. The approach is one of being on the offensive, to show that the wisdom of the world is complete foolishness. You do this by answering a fool according to his folly. Step into his shoes and look out from his eyes, from his ultimate presupposition or world-view, and point out the internal inconsistencies. Reductio Ad Absurdum - Reduce to absurity or "tweak with their epistomology" as I like to say it.You then explain how only the Christian presupposition "in the beginning God" is self-attesting. It's the only world-view that is internally consistent. Without "in the beginning God" we have no reasonable reason to believe in reason, no logical basis to believe in logic. As Augastine said, "We believe in order to know". Or as the proverb says, "The fear of God is the beginning of wisdom". So you prove the Christian world-view because of the impossibility of the contrary.Offensively show how all other ground is sinking sand except for the solid rock of Jesus Christ. This biblical approach rips the carpet out from under their feet. It shows them how they have firmly planted their feet in thin air and that they are sawing off the branch they are sitting on.So I came up with a question for a relativist using the internal critique method:"if truth is relative, then could this truth, that you say is true, that all truth is relative be relative itself and thereby not always be true?"-----------I recommend that every Christian read "Always Ready" by Greg Bahnsen
