A commenter asks why I don’t endorse the claim that the transcendental argument for the existence of God (TAG) gives us epistemic certainty (which I take to mean that the argument delivers a conclusion that has maximal epistemic warrant and could not be rationally doubted). After all, if TAG proves Christian theism “by the impossibility of the contrary”, as many of its advocates have claimed, wouldn’t it follow that TAG’s conclusion is epistemically certain?
As a preliminary matter it should be pointed out that the epistemic status of normal Christian belief doesn’t depend on TAG. We should distinguish between the means by which we know that Christianity is true (in my view, the external testimony of Scripture coupled with the internal testimony of the Holy Spirit, as per chapter 1 of the WCF) and the means by which we show (to others) that Christianity is true. TAG serves the latter purpose, not the former. The “full persuasion and assurance” of our faith has nothing to do with TAG — and it’s a good thing too, since TAG was all but unknown before the 20th century. I make this point only because some Van Tilians have given the impression (usually inadvertently) that the certainty of our knowledge of God and the gospel somehow hangs on the cogency of TAG.
Turning now to the claim that TAG delivers epistemic certainty about Christian theism, I offer three reasons to question that claim. First, as I’ve suggested elsewhere, TAG is best understood not as one unique argument but as a family of arguments with a common theme or goal. So when someone claims that TAG delivers epistemic certainty, my first response is to ask, “Which TAG? Which formulation of TAG?”
Secondly, I’m unaware of any formulation of TAG that proves specifically Christian theism; thus it’s hard to evaluate the claim that TAG proves Christian theism with epistemic certainty.
Thirdly — and this is the most important point — we need to be clear on what it would take for any argument to deliver its conclusion with epistemic certainty. The strongest type of argument is a deductive argument, in which the conclusion follows necessarily from the premises: if the premises are true then the conclusion must be true. But even in a deductive argument, the conclusion cannot enjoy greater epistemic warrant than the premises (at least in the case where the warrant for the conclusion is taken to derive from the argument itself). What this means is that TAG can deliver epistemic certainty only if all of its premises are epistemically certain.
The problem, however, is that the transcendental premise of TAG (that God’s existence is a metaphysically necessary condition of some essential feature of human thought or experience) isn’t self-evidently true and it requires quite sophisticated argumentation in its support. But the more sophisticated an argument, the more room there is for doubt about its cogency — even the doubt that we have adequately understood all of the premises and inferences — and the less plausible it becomes to maintain that all of the premises of the argument are epistemically certain. In the case of TAG, it stretches credibility to claim that every premise of the argument (and of the sub-arguments given in support of the top-level premises) is epistemically certain.
None of this implies that TAG is a failure or a pipe dream. But Van Tilians do themselves no favors by needlessly overstating what TAG can (or could) accomplish. To my mind, the relevant question here isn’t “Does TAG deliver epistemic certainty?” but rather “Why think that TAG should deliver epistemic certainty?”