Actually, this is a very brief take on apologist/theologian/philosopher Cornelius Van Til's work as contained in the readings and interpretation found in Greg Bahnsen's massive tome, Van Til's Apologetic: Readings and Analysis. All in all, I'm very sympathetic with a lot of Van Til's ideas. I think he gets better than most apologists how the way we react to, interpret, experience, filter, and reason about ourselves and the world around us is in large part dependent on what we already believe (philosopher W.V.O. Quine and others have made some headwork with this idea), particularly our most fundamental beliefs or assumptions - and that Christians and non-Christians come to the world with different sets of these. I also appreciate the idea that sin affects this set - it has real consequences for the way our minds work - and that our knowledge of God is based not primarily on reasoning or experience but on God's own testimony (we have, as made in the image of God, a sensus divinitas).
So far so good - when Van Til (and Bahnsen, who substantially agrees with Van Til) goes beyond all this, however, it hard to follow what the reasoning is supposed to be. Van Til thinks that the only appropriate apologetic method is to use a transcendental argument to the effect that only on the presupposition of Christianity is reasoning or pretty much anything else possible at all. Here's where things start to get messy. Sometimes it seems like Van Til is saying that unless a person already assumes Christianity, they cannot make sense of any of this stuff. Other times, it seems like he is saying that unless Christianity is true, none of this stuff would be possible. These are two distinct claims, but he seems to slide back and forth between the two without noticing and this creates a lot of problems with some of the arguments in favor of his method and against other apologetic methods. Most often, he seems to slide back and forth, equivocating between metaphysical and epistemological senses of various terms or concepts, again making for potentially fallacious argumentation. There also seems to be some equivocation relating to other terms such as "authority" or "primacy". Then there's the claim that there are no neutral beliefs - one either presupposes Christianity or its opposite. His claim is that to the extent that a non-Christian agrees with Christianity on some fact, he or she is unwittingly (and inconsistently with his or her own position) presupposing Christianity, an idea which seems to depend on the successful implementation of his transcendental argument (and which, unfortunately, inherits the same ambiguity which then affects his arguments against opponents).
Unfortunately, Van Til (and Bahnsen) does not do a lot to actually show that the transcendental argument works. Simply saying that only on the presupposition of Christianity is, say, reasoning possible does not show that it is so. We need more argumentation. Unfortunately, not much is forthcoming, and what is provided tends to contain gaps in reasoning that are (again, unfortunately) not filled. Over and over again, claims are made as to what the non-Christian is committed to with little in the way of proof that he or she is actually so-committed. This also infects arguments against other methodologies (not to mention some of the mistaken or at least controversial interpretations of various historical philosophers). To take but one instance (my own comments are in brackets), Van Til claims that traditional methods are "allowing for an ultimate realm of 'chance' out of which might come 'facts' such as are wholly new for God and for man. [Where do they do this? How? Is this really a good interpretation?] Such 'facts' would be uninterpreted and unexplainable in terms of the general or special revelation of God. [Why? How does this follow?]" I won't even start on the claim that the use of logic in traditional methods of defending Christianity puts logic above God or in control of God or makes God not God, etc. (There are many things wrong here, one being that Van Til seems to assume without argument that the facts of logic are things out there to which God might be subordinated, whereas many philosophers (not all) would deny that such that there are facts of logic at all in a metaphysical sense - the law of non-contradiction is, on such a view, necessarily true but without some unique entity out there making it true since describing substantive reality is not even what the statement is supposed to do in the first place)
I sometimes had similar problems with the other presupposionalist book I read recently, Vern Poythress's book on logic, which, in its statements and arguments, pretty clearly confused logic with reasoning over and over again and explicitly stated that logic is something like a codification of rationality, which it is not. In any case, I was a bit dissapointed with the argumentation of the presuppositionalist writings I have read so far, despite agreeing with a fair bit as well. I have some other books along the same vein lined up to read (including more Van Til and Bahnsen), so I am hoping that there is more to some of these arguments than I have already seen.