The issue is the context of the statements Bahnsen quotes.
You state, "I deal expressly with the immediate surrounding context of Clark's quotes. Read a bit further into the critique before passing final judgment on this point." "This point" is the quote from Clark that Bahnsen claims is a departure from Van Tillian-presuppositional apologetics. I read your second post and you do not continue discussing it. I will read through the rest of the posts to see if you deal with it in detail.
I will grant my visitor's point that in the quoted text Clark's approach is a departure from Van Til's presuppositionalism. But in granting that, I mean to emphasize that the departure is, I believe, superficial. More on that below.
Now you did provide more context in you last comment to me. You say, "Of course they aren't presuppositional in the sense of presenting a positive argument. But that is irrelevant because the entire point is that Clark is performing an internal critique of opposing views, which requires the arguer to adopt the principle of the opponent to demonstrate the contradiction: Modus tollens. As your second question alludes, Clark is, in every one of the quotes that Bahnsen criticizes, performing internal critiques."
I see this as a departure from Bahnsen. In doing an internal critique, Bahnsen would not say something like "Chistianity in particular furnishes a BETTER method than secularism is a POSSIBILITY not to be dismissed." The presupposition that Bahnsen targets in internal critiques is not the unbeliever's adherence to Modus tollens but to his rejection of Scripture as revelation. The non-christian rejection of Scripture undermines logic and reason in totality, not in possibility. Bahnsen is not after contradictions here and there. He is out to show that apart from God there are no contradictions period. There is no modus tollens.
Performing an internal critique of opposing views does not require the arguer to adopt the CONFESSED principle of the opponent. It requires the arguer to confront him with the consequenses of his UNCONFESSED principle: God and His Word.
I think my visitor has created a false disjunction between modus tollens and the confrontation of revelation. Modus tollens, a logical argument that adopts a premise on the basis of demonstrating the absurdity of its implications, does not actually entail the assent to that adopted premise. Indeed, if one assented to the premise while knowingly attempting a modus tollens, one would be setting oneself up for contradiction!
The real difference between Bahnsen and Clark, or even Van Til and Clark, is simply a matter of method, and not of presuppositions. Clark, Van Til, and Bahnsen all presuppose that Scripture is the Word of God and is the only justification for knowledge. Bahsnen presses Clark because of his choice of "possible" or "probable" or other uncertain language, but really the difference is rhetorical rather than substantive. Clark wishes to engage the unbeliever in a discovery, whereas Bahnsen and Van Til wish to provoke their spirit by proclamation.
Clark does not hide his presuppositions from his readers, but he does not assert them with the force of Bahnsen or Van Til. For example, after stating Augustine's position he concludes:
"With the Bible and its theology Augustine's view of history can stand; without that theology Popper's strenuous moral pleading, with which he closes his book, has no foundation." (p. 231 Historiography: Secular and Religious)
Clark here affirms the basic presuppositional argument: apart from Revelation, no foundation for history can be had. While Clark takes 230 pages to reach this statement, it is not inconsistent with his professions of "probability," for in those places he is still canvassing views to indirectly lead his reader through the failures of unbelieving positions (modus tollens, or disjunctive syllogism, which we'll come back to).
Now one may certainly argue that Bahnsen's straightfoward approach is more keen to his liking, and one may even argue that Bahnsen's approach provides more confidence to the believer in his defense of the faith, for it places the critical issue at the forefront. However, to accuse Clark of contradiction is simply fallacious. The language of possibility in a modus tollens does not imply that the arguer is denying what he really wishes to prove. It merely shows that the unbeliever cannot establish what he would like to establish: the impossibility or improbability of Christianity.
Here is the full quote that Bahnsen pulls from Clark's chapter on Augustine (I've emboldened Bahnsen's excised portion):
In Part One the discussion on a few occasions approached, if it did not trespass on, matters of religion. For example, the question whether historians should pronounce moral judgments on great men requires for an affirmative answer an epistemological method of justifying a moral norm. That religion or Christianity in particular furnishes a better method than secularism is a possibility not to be dismissed without discussion.
While Clark is now introducing Christianity into his treatise on historiography, he is still engaging in a disjunctive syllogism, which aims to remove alternatives prior to asserting Christianity as the valid choice. While this is not Van Tillian in nature, it is still presuppositional, for one cannot eliminate on the basis of neutrality, but must have a foundation upon which to argue, which Clark freely admits is Christian Revelation.
On page 264-66 of his book, Bahnsen claims that Clark falls prey to the logical fallacy of asserting the consequent in trying to prove the validity of Christianity, and then, in order to salvage the argument, would have to use a disjunctive syllogism that can only be proven by omniscience, since the disjunction requires EVERY possibility to be dismantled in order to prove only one.
However, in his reply to Arthur Holmes in the edited volume, The Philosophy of Gordon Clark (a chapter Bahnsen cites), Clark expressly details his use of the disjunctive syllogism, and how it comports with his presupposition. First, he explains the usefulness of the disjunctive syllogism (notice the denial of neutrality):
It is the emphasis on system that justifies my use of the "disjunctive syllogism." Since in a logical system all the theorems come from the axioms, and from nowhere else, since indeed the meaning of the theorems depends on the axioms, a particular theorem cannot be found in two different systems. When such a theorem seems to occur, either there is an inconsistency in one or both systems, or the systems overlap--in which case at least one of them is not a universal system. Therefore, the so-called "disjunctive syllogism," the denial of a middle ground, and the principle that "he who is not for us is against us" are logical necessities. (p. 431)
He then goes on to assent to the fact that his disjunctions do not undertake to refute every alternative, but then he argues that many particular alternatives fall under his criticisms of empiricism, because of their basic (and unproven) assertion that physical sensation is a requirement for the possession and/or justification of human knowledge. He follows with more defenses of his disjunctions, then comes to another important presuppositional point:
In my debates with some who deny it, I have maintained that Christians and non-christians have certain "common ground." That is to say, a regenerate and unregenerate person may believe the same proposition [e.g. the sky is blue today]. But this by no means implies that a given proposition can be deduced indifferently from Christian and from secular presuppositions. Hence, the statement [by Holmes] that "We can learn about the form of logical reasoning, therefore, from non-christians philosophers," misses the point. In the case of logic the unacceptability of secular logic becomes clearer than in the case of Aristotle when Dewey brings logic into a more consistent connection with his secularism by denying logic's finality and arguing that the principles of logic, like the principles of grammar, change with use from age to age. Therefore, I should disagree with the idea that we can learn logic from Dewey. The principle involved in my argument is that incompatible axioms [presuppositions] do not imply identical conclusions. If the words sometimes sound similar, the intellectual content is not. (pp. 435-36)
In this quote Clark is denying neutrality, asserting the incoherence of secularist epistemology, and establishing that the Christian system is incompatible with all others. He obviously holds to the Christian system in arguing against all others, and is therefore consistent in accepting it as his presupposition.
There is a great deal more to be gleaned from Clark's reply to Holmes, much of which addresses other objections that Bahnsen makes to Clark. I remain puzzled as to how Bahnsen could have missed the cogency of Clark's replies. A summary may perhaps whet my readers's appetites to investigate further:
1. Bahnsen objects to Clark's use of "autonomous" logic as a means of "validating" Scripture. But Clark replies that Paul uses logic "at least on par with that of Aristotle," and since logic is evinced by Scripture, it is part of God's revelation, and not some autonomous system. In his logic textbook, he gives an example of Paul using an enthymeme (pg. 3), and on pg. 119 he identifies several logical forms found in Scripture:
For example, Romans 4:2 is an enthymematic hypothetical destructive syllogism. Romans 5:13 is a hypothetical constructive syllogism. 1 Corinthians 15:15-18 is a sorites.
If Paul uses logical forms, and Paul is inspired by the Holy Spirit, then we know for certain that God exhibits a logical mind. How then can logic be atheistic or autonomous when used to evidence the consistency of God's Word? Note well that I used the word "evidence" and not "prove." I've discussed in another post already where Clark openly asserts that Christianity is only "proven" by a change of mind in the individual brought about by the Holy Spirit.
So Bahnsen falsely accuses Clark of asserting the consequent, for Bahnsen's alleged reproduction of Clark (1. If the hypothesis of Christianity is true, then X. 2. X is the case. 3. Therefore, Christianity is true) is not Clark position at all. The second readjusted disjunctive syllogism that Bahnsen attempts to undermine also falls flat because Clark's use of the disjunction is not a denial of Christian presuppositionalism, but is rather built upon it. So far, argues Clark, in the history of philosophy there have only been four basic epistemologies offered: 1. Rationalism, 2. Empiricism, 3. Irrationalism and 4. Revelation. His disjunction does not treat all individual manifestations of these views, but rather he does complete a disjunction of the broad categories of rationalism, empiricism, and irrationalism. The burden of proof is upon individual views to establish epistemologies that do not fall under the three broad categories Clark destroys. Merely asserting alternatives is not enough (as Clark's rejoinder to Holmes also labors to show).
To conclude, Clark was every bit as much a presuppositionalist as Van Til or Bahnsen ever were in their commitment to the Revelation of Christ, God's Word inscripturated. The major distinction comes in the direct vs. indirect method of exposing the folly of unbelief. Bahnsen and Van Til are direct, whereas Clark is indirect. And actually, in his doctrinal writings, Clark is much more explicit and direct.