But there is perhaps a better way to frame the argument. After all, A presupposes B is not the same as A implies B.
Human predication presuppose God
/ Human predication * ~Human predication
This latter argument form demonstrates that in order for any truth value to be assigned to human predication or no human predication, God must be presupposed. The transcendental argument is saying that if there is no God, there is no truth value where human predication is concerned. All this is simply to point out that there is a difference in traditional arguments which trade on implication and transcendental arguments which trade on presupposition. Collect is helpful in summary: "However, if God's existence is a necessary condition for the both the truth or falsity of causality, then denying God's existence while results in a failure to predicate anything at all." [Don Collett: Van Til and Presuppositionalism Revisited. See also Strawson, An Introduction to Logical Theory]
Now, Bahnsen would call it the impossibility of the contrary, but by contrary he means contradictory. Additionally, that this is an argument is, as Mike Butler puts it, beyond debate. Whether it is a good argument is a different matter. To answer Dawson’s charge of circularity, however, is not too difficult. As Craig admits, there is more to it than that. TAG is an epistemological transcendental argument. Characterizing it as having vicious circularity or begging the question simply means that one does not truly understand what TAG is doing.
In fact, I would say that Dawson Bethric's grasp of what Van Til was doing and Bahnsen after him, by employing a transcendental argument for is terribly confused. As Collet rightly points out in his excellent paper, Van Til and Transcendental Argument Revisited, Van Til was concerned to make sure that Christians employ the sort of apologetic argument that preserves the logically primitive and absolute character of God's existence. This can only be done by starting with the premise that God's existence is the necessary precondition for argument itself. That's right, God's existence is the necessary precondition for argumentation itself. What this means is that the concept of God should function as a logically primitive proposition rather than a logically derived one.
What Dawson never seems to deal with is the "man behind the curtain" of Presuppositional Apologetics. What does that mean? It means that Dawson does not interact much, if at all, with the doctrines of divine aseity and transcendence. If he did, perhaps he could connect those dots. What Dawson, and many, many others fail to understand is that one cannot truly understand Presuppositional Apologetics unless they understand why it exists and what it seeks to accomplish. Van Til's apologetic is designed to protect reformed doctrine and more precisely God's self-contained, independent, and transcendent nature. If this doctrine is correct, and surely it is, then no axiom can be more ultimate than God's existence. Hence, traditional approaches unwittingly argue for God's existence as a logically derivative status, elevating other principles to an unacceptable primitive status inconsistent with basic Christian doctrine.
Once again, Don Collett is helpful: "Indeed, one may go further and raise the question whether finite creatures can begin any argument without making assumptions of some sort or other. The real question is not whether initial assumptions can be avoided, but whether subsequent argument confirms their soundness."