They're a rather strange lot. The central tenet of their ideology is that logic, intelligibility, and the meaning of evil presuppose the existence of their god. That is, that without their god, one cannot define these terms.
Wikipedia has an entry for presuppositionalism that says this:
The key discriminator of this school is that it maintains that the Christian apologist must assume the truth of the supernatural revelation contained in the Bible (that is, the Christian worldview) because there can be no set of neutral assumptions from which to reason with a non-Christian. In other words, presuppositionalists say that a Christian cannot consistently declare his belief in the necessary existence of the God of the Bible and simultaneously argue on the basis of a different set of assumptions (presumably those of the non-Christian) in which God may or may not exist.
By way of contrast, the other schools of Christian apologetics assume the world is intelligible apart from belief in the existence of God and argue exclusively on (purportedly) neutral grounds to support trusting the Christian Scriptures.
As far as I can tell, the presuppositionalists just want to build a wall around their beliefs that will isolate them from argumentation. Still, I have been having fun in the debate, despite my frustration with their inability to formulate an argument that makes any sense to me.
I enjoyed writing my last response so much that I've decided to post it here. Um, also, I haven't had time to write any real posts recently. :)
To a presuppositionalist:
Suppose we're playing Blackjack at Honest Joe's casino. No cheating has ever been recorded at Honest Joe's.
The dealer has 10 showing. Both of us have a 7 and a 2. Should you and I take another card?
We cannot prove that our next card will be sufficient to beat the dealer. That's fair, this is a game of chance, after all.
However, we also cannot prove that the deck is not stacked against us (e.g., a casino employee has manually selected our upcoming cards to be deuces and 3's). Though no cheating has ever been recorded in this casino, this fact does not constitute total proof that no cheating will ever occur.
Yet, we both know that if we don't take a card, we are guaranteed to lose.
I don't need absolute certainty that the dealer has not stacked the deck against me. No faith is required. Simply put, I have no chance of winning if I don't take a card, and at least some chance otherwise. Logically, I should take a card. I ask the dealer for another card.
In contrast, you demand absolute certainty that the game isn't rigged. No person at the table, including casino security, can tell you with total, unerring certainty that the game is fair (hey, it could be an inside job).
Since no measurable entity can guarantee fairness, you invent an unmeasurable one. You theorize that there is an invisible Inspector who guarantees the fairness of the game. Though the Inspector is totally invisible, you claim that he must exist if for no other reason than to guarantee fairness of the Blackjack table. Furthermore, you claim that it would be impossible to even play Blackjack without the existence of the Inspector.
Finally, with your self-assured certainty in the existence of the Inspector, you tell the dealer that you want another card.
We're both dealt 10's and we both beat the dealer.
This is a good analogy for our debate so far. As this analogy shows, there is no need whatsoever for the Inspector to exist at all.
You may argue that the risk of fraud is too high at the casino, but I would answer that it's the only game in town, so we have to live with it, fair or not.
When asked how we might otherwise know of the Inspector's existence, you answer by saying that he will reveal himself in rare, miraculous cases of... cheating! Indeed, some players have reported cheating in the past (e.g., they claimed to have seen the dealer's up card change during a hand), but whenever the casino security tapes are reviewed, no evidence of cheating can be found.
Of course, the premise about the Inspector guaranteeing fairness of the game is really just a smokescreen. The real reason that the Inspector was invented was to guarantee that players can win when they leave the casino. It is said that those who leave a tip for the Inspector are guaranteed a net payoff on their way out of the casino. Curiously, no one ever gets out of the casino alive, and the Inspector never collects his tips.