What's presuppositionalism?

For years there's been controversy over the correct interpretation of Van Tilian apologetics. I don't comment on this very often because I think it's usually a blind alley. 

What accounts for persistent disagreement regarding the interpretation of Van Tilian apologetics? I'm reminded of what the SEP entry on the double effect principle says: "It is not at all clear that all of the examples that double effect has been invoked to justify can be explained by a single principle."

And that may be a large part of the difficulty in pinning down Van Tilian apologetics. Perhaps it's not the outgrowth of a single overarching principle, but a family of related positions. Or maybe they're not all closely related. Maybe some elements are adventitious.

1. TAG

Considered in isolation, even though it's associated with Van Tilian apologetics, and sponsored by Van Tilian apologetics, as if that's a distinctive of Van Tilian apologetics, there's no reason why TAG couldn't be just one among a range of a priori and a posteriori theistic proofs. No reason, at this discrete level, that it couldn't be incorporated into classical  apologetics or figure in a cumulative case approach. 

2. The necessity of TAG

If, however, we take a step back and ask why TAG is said to be necessary, or why transcendental arguments generally are important or indispensable, then at that underlying level it's not just one of many theistic proofs. Rather, Van Til's contention is that we naturally take many fundamental truths for granted that are groundless unless God exists. And not mere theism, but Reformed theism. 

On that broader and deeper level, the claim is that TAG reflects a distinctive, all-embracing, and unifying orientation regarding the justification of knowledge. Without that theistic grounding, global skepticism looms large.  Even if TAG is compatible with classical theism, or a commutative case metrology, the rationale for TAG is more foundational. As the IEP entry puts it, "Transcendental arguments are partly non-empirical, often anti-skeptical arguments focusing on necessary enabling conditions either of coherent experience or the possession or employment of some kind of knowledge or cognitive ability, where the opponent is not in a position to question the fact of this experience, knowledge, or cognitive ability, and where the revealed preconditions include what the opponent questions. Such arguments take as a premise some obvious fact about our mental life—such as some aspect of our knowledge, our experience, our beliefs, or our cognitive abilities—and add a claim that some other state of affairs is a necessary condition of the first one."

On this view, even if there's nothing distinctively presuppositional about TAG, there is something distinctive about transcendental theism.

3. Reductio ad absurdum 

In addition, Van Til had a two-prong strategy for apologetic dialogue or analysis: assume their viewpoint for the sake of argument and take it to a logical extreme; have them assume the Christian (i.e. Reformed) viewpoint for the sake of argument and take it to a logical extreme. Compare and contrast their respective explanatory power or reductionism. A reductio ad absurdum or argument ad impossibile. 

(3) is related to (2). As a Calvinist, Van Til thought that for experience to be coherent, everything must happen for a reason. Every event must be coordinated in a part/whole, means/ends relation, according to a wise and benevolent master plan for the world (predestination, meticulous providence). By contrast, theological indeterminism leads to loss of ultimate coherence. Uncontrolled, uncoordinated events that are individually pointless, going nowhere. 

4. Divine incomprehensibility 

Due to his interpretation of divine incomprehensibility, Van Til didn't think it was possible to prove God directly. His intuition seems to be that if God is paradoxical, then he defies straightforward proof. 

There are other components to his overall thinking, but those are crucial features, I'd say. Is this a tight package? If you accept (2), then that commits you to (1). On the other hand, you could see the value of (1) without strong commitment to (2). 

Likewise, belief in (4) commits you to (1), and perhaps to (2), but you can see the value of (1) and or (2) without a strong commitment to (4). 

(3) is a practical strategy rather than a principle, although (3) may be a way of illustrating the contrasting alternatives implicit in (2). 

Another issue is whether transcendental arguments are, in fact, a unique kind of argument. According to the SEP entry, 

As standardly conceived, transcendental arguments are taken to be distinctive in involving a certain sort of claim, namely that X is a necessary condition for the possibility of Y—where then, given that Y is the case, it logically follows that X must be the case too. Moreover, because these arguments are generally used to respond to skeptics who take our knowledge claims to be problematic, the Y in question is then normally taken to be some fact about us or our mental life which the skeptic can be expected to accept without question (e.g., that we have experiences, or make certain judgements, or perform certain actions, or have certain capacities, and so on), where X is then something the skeptic doubts or denies (e.g., the existence of the external world, or of the necessary causal relation between events, or of other minds, or the force of moral reasons).

https://plato.stanford.edu/entries/transcendental-arguments/

But couldn't some other theistic proofs be framed in similar terms? They take some generally uncontested fact like the existence of the physical world, or thinking beings, then give reasons for supposing that God supplies a necessary condition for their existence. Cosmological arguments give reasons for why God supplies a necessary condition for the possible and actual existence of the universe. Teleological arguments give reasons for why God supplies a necessary condition for certain types of natural organization. The moral argument gives reasons for why God supplies a necessary condition for moral realism. The argument from reason and argument from consciousness give reasons for why God supplies a necessary condition for consciousness and the reliability of reason.

To be sure, some people deny moral realism, &c., but then you just recast it in hypothetical terms: If moral realism is true, then that it must be grounded in God. If mathematical realism is true, then it must be grounded in God. If modal realism is true, then it must be grounded in God. The existence of something necessary is a prerequisite for the existence of something contingent. And so on and so forth. 

Perhaps they are treated as distinctive because, as originally conceived, they are epistemological theistic arguments. But is the epistemological application an exclusive kind of argument or a specific application of a more general principle?