How do you know what you know? Could you be wrong about everything you think you know? Is it viciously circular to use your reasoning to justify your reasoning?
Familiar with these types of questions? I sure wasn't, and found myself facing them recently. I was baffled that I would be facing something as ridiculous to begin with, and yet facinated at the same time at the use of this presuppositional apologetic. As I've said, the questions are nonsensical, and yet the individuals asking them think they are asking the real hard hitting questions that atheists and non-believers can't answer.
Foundationally, I guess it would be fair to say that the whole argument hinges on the following assertion: without God (and the Christian god, of course), you can't know anything. Those most noted for this style of argument (you may already know of them) are Eric Hovind, and Sye Ten Bruggencate. What I find interesting about this argument is the almost clever way it establishes early on in the debate or conversation not only traps to later spring on the other, but also really convenient ways of allowing the presuppositionalist a way out of having to answer a very pointed question or a very tough one. What else is interesting is that the complaints one would have about the bible (and I'm pointing specifically at Christianity for this blog because that's the religion on which these presuppositionalists hang their hat), those arguments can also be dodged with an admission that they fully expect a non-believer would consider them crazy for believing things in the bible like talking snakes and donkeys, teachings on rape and slavery, and the like. Without God those things simply make no sense. But because they have a God, for whom all things are possible, the seemingly impossible is then suddenly explainable. Can't explain how slavery is moral? Insert God, and the seemingly impossible task of making slavery a moral thing can suddenly be explained. However, it is rare, it seems, to get into a conversation with a presuppositionalist on these types of issues. Sye Ten Bruggencate will shrug it off as an interesting topic of discussion for a bible study, but that he isn't about to get into a bible discussion with a non-believer. But! If you're willing to accept the bible as true (effectively becoming a Christian, and conceding defeat) then he would be happy to discuss those topics with you. I seriously doubt this conversation would take place, at least not in any meaningful sense. All he would have to say, if you maintained your objection to the immorality of rape or slavery being sanctioned, your conversion wasn't a true one, and attempts at engaging in that conversation further would halt. It's nothing more than an attempt to avoid answering or addressing the real problems people have with these texts.
So, let's unpack the presuppositional apologetic a little bit further. Debates that I have seen thus far almost all follow the same pattern when this apologetic is employed; the theist will make a claim that they can prove to you that God exists (again, the Christian god), then proceed to ask you questions, the openers often being one mentioned above. But this is the linguistic trapping I was talking about before. I'm no linguist, so I'm not here to teach the subject, nor could I begin to, but it's important to note that the actual linguists this style of argumentation has been used on has ultimately failed. They were able to point out what a layperson would almost certainly miss, and that is the switch in meaning of a single word they will use in their questions, such as 'knowledge.' For a great example of this, look for the debate/discussion between Sye Ten Bruggencate, Eric Hovind, and Dr. Michael Jones, Bob Greaves and Greg Brahe on a show called "The Place." It's a lengthy conversation, but if you're interested in learning more about this style of apologetic it's worth the investment of your time.
Sye and Eric are inconsistent in their meaning, but it's the switch in meaning from their opening questions to their follow-up uses that actually gives them their leg to stand on, and confuse people to begin with, and trick them into following them along in their line of reasoning. At first, it would have been fair to say that, not being linguists themselves, they may not have been aware that they were even doing this, but after having it pointed out to them (and several times at that), they continued to make the same substitutions. There were times when the switch was being pointed out to them where they would dodge the question by playing their conversational dodgeball games eluded to earlier, saying things like, "could you be wrong about that," or, "how do you know that?" These questions are designed to distract from the topic at hand, evade answering an opponent's questions, and redirecting the conversation topic to the "quicksand" these apologists love. These ploys effectively halt any meaningful discussion, which leads me to conclude that they are not designed really to convince non-believers, but more intended for those already in their 'camp.'
Perhaps another fair way of rephrasing their argument would be to say that they take advantage of the position of the unbeliever not having a god, and then posit that because they account for their knowledge because of the god they claim exists, it therefore follows that the atheist cannot account for their knowledge by any other means than borrowing from the theist's worldview.
My experience with a presuppositional argument was just recently, and I was just as surprised to hear the silly question of, "how do you know what you know," but the inability to accept my reasonable answer. My mind immediately went to evidence, and something that is demonstrable, and used the simplest example that came to me, and stated that others have demonstrated to me that two plus two equals four, and that I can test it, and repeat it, and verify it to be accurate and consistent. This would certainly be uncontested. Well, no. It, like many other answers to these apologist's ridiculous questions, is repeatedly met with the, "how do you know that," ad nauseum.
Trying to bring the conversation back to a place where both parties can take part in a reasonable conversation would seem impossible, but it's not a failing on the non-believer's part I would argue. The very foundation of the presuppositional position is that of, "I'm right, you're wrong, therefore this conversation's only natural end point is where you finally see that this is so." Once this is realized, it immediately ceases to be a conversation or debate. It might be wise to walk away at this point, particularly if the individual happens to be a friend or family member, and it just isn't worth losing that relationship to you. Not only is this style of argument extremely annoying, but also extremely arrogant in it's nature, and will ultimately, in my opinion, escalate to a heated exchange.
In closing, if you're interested in learning more about this apologetic, the subtle traps it lays, and the refutations for it, I would recommend a few sources. I mentioned earlier The New Covenant Group, which is a panel of sorts that includes both atheists and theists, and tackled this issue at least a few times in great detail, and is a fantastically comprehensive guide to understanding this apologetic. Also of note, are Dan Courtney, and KnownNoMore. All are available on YouTube.