I have noticed as of late that there is an increase in discussion amongst atheists about the subject of presuppositions. I think this is just great. After all, one of the most difficult tasks in debate (formal or otherwise) with unbelievers is getting them to understand the role that presuppositions play in their thinking. We've talked about this here at Choosing Hats in great detail, both in posts and in our Bible Study on Bahnsen's "Always Ready". This is the good news.
The bad news is that I do not believe that these same atheists understand how completely foundational these very presuppositions are to their reasoning process. Despite their concurrence that such things exist, and their commentary about the role they play, the atheists that I have read or listened to fall prey to the same thing many Christians do - assuming the very presuppositions they are defending, without the realization they are doing so.
Presuppositions exist at many levels, but it is the foundational ones (those which are most basic) that I am interested in highlighting here. It is just the nature of these particular presuppositions that makes them impossible to step outside of while evaluating them. For the Christian, the most basic of all presuppositions is the existence of God. That means that logically speaking it is not possible for a Christian to evaluate anything at all without ultimately presupposing God, including the belief that God exists. This is, after all, the very source of the complaint of circularity against those presuppositionalists who employ TAG.
The unbeliever has their basic presuppositions, too. One of the most basic is the belief that they are able to reason without a foundational appeal to the God of the Bible. It isn't that they necessarily deny God's existence directly as part of their reasoning process, but rather it is the fact that they presume to even question whether God exists at all. Doing so implies that they believe it is possible to know at least one thing (whether or not God exists) without ultimately relying upon God to answer that question.
Van Til uses the analogy of a telescope, where the telescope is God and the star is any fact that a person wishes to investigate. The epistemologically self-conscious Christian will always look through the telescope anytime they wish to investigate a "fact". The unbeliever on the other hand attempts to look directly at the star without the aid of the telescope, thinking they will be able to have an accurate view of the fact. The real problem is uncovered when the "star" in question is the existence of God.
The unbeliever assumes that there is no telescope that is required in order to determine whether there exists a telescope which is required to "see" any "fact". They attempt to look directly at the "fact" of the star in order to see whether or not there is a telescope which is required in order to see any facts at all. The problem is self-evident. If the Bible is true and such a God as this exists, the unbeliever is never going to conclude that such a God exists simply by looking directly at the "stars" (i.e. using un-aided human reason).
This is, to me, the clearest example of a foundational presupposition that no matter how hard they try, the atheist cannot *logically* "put aside" in order to question whether or not God exists.