Bahnsen's Use of Modus Tollens and Modus Ponens

It was recently asked of me why Greg Bahnsen argued transcendentally using Modus Tollens (MT) as opposed to Modus Ponens (MP), which I suppose is a fair question since even John Frame, it would seem to me, did not seem to see any persuasive reason why Bahnsen was a stickler about using one formulation rather than the other.

To argue by way of negation as opposed to affirmation is much more powerful because it reduces the opposing worldview to absurdity. There’s a reason, after all, why MT is referred to as a reductio ad absurdum argument. Both forms (MT and MP) are valid forms of argumentation; can be converted to the other; and argue to the same narrow conclusion – in this case that God exists. But it is one thing to say that causality logically implies God, which is what MP achieves, and quite another to argue that without God there could be no causality. The latter, of course, employs MT. At the very least, there is a tactical difference between: “Ccausality, therefore, God” and “No God, therefore, no causality”.

Consider the major premise: “If logic (predication, induction, or whatever), then God.” I’ll now try to show where we might use MP and where we should use MT to be most affective given that one major premise, with which both MP and MT may begin. First I’ll address MT.

The reason Bahnsen argued by way of negation (MT) in all his formal debates and in his classroom lectures on transcendental arguments is because he appreciated that no other formal construct illustrated more clearly and powerfully the point he wanted to make, that intelligible experience has no rational grounding given the negation of God’s existence. Bahnsen, following Van Til, wanted to argue that to predicate anything God must first exist (i.e. predication presupposes God). The way he chose to demonstrate that was by assuming for argument’s sake the opponents position that God does not exist, which required the negation of the consequent of the major premise, as opposed to the affirmation of the antecedent. By assuming the atheist’s worldview, Bahnsen for argument’s sake would negate the consequent of the major premise, “God”, which forced the conclusion that there is no logic (for instance). The reductio was apparent since logic was being presupposed by the one arguing the opposing position. By negating the consequent of the major premise the opponent is left to deal with the force of the conclusion: no predication – yet while all the time predicating! Bahnsen’s mantra was that to argue against God one must first presuppose God in his reasoning. Accordingly, he would argue by way of reductio: “No God”, which affectively led to the absurd conclusion of “no possibility of argumentation.” Bahnsen said over and over again that “the proof of God’s existence is that without him one could not prove anything.” That is precisely why he negated “God exists” in the minor premise - to show the absurd conclusion that there is no logic, etc.

Where MP might be found useful is in the apologist’s follow-up, which is to offer the solution to the quandary. Once one has been left with the absurd conclusion of “no logic” and cannot make logic comport with his unbelieving worldview, then by all means it is under good regulation to reformulate the argument with a new minor premise, that being the assertion of the antecedent – “Logic” (or causality, or prediction, or induction, etc.) In other words, once one is left with the conclusion of the reductio, which is the conclusion that his unbelieving worldview leaves him with(!) - it might be a good idea to assert the antecedent of the major premise (logic) in order to arrive at the conclusion: “God exists”: If logic, then God; ~God, therefore, ~logic (reductio). Ah, but there is logic (assert antecedent of major premise), therefore, God (MP). That is the only use of MP one will probably ever find in Bahnsen’s lectures, debates or writings with respect to this particular matter. MP is employed to give the solution to the dilemma. But even that can be done by MT! In other words, the conclusion of the argument is essentially "~God, therefore, logic". Accordingly, by treating that as a premise, we can simply add to the argument by negating logic in order to affirm God, which is to continue the argument with a form of MT rather than applying MP to the original major premise. In other words: If logic, then God. No God, then no logic. Logic, therefore, God.

With his debate against Dr. Stein, Dr. Bahnsen argued as his major premise: “If the uniformity of nature, then God” – (which was to posit the assertion that the uniformity of nature presupposed God’s existence.) The negation of God’s existence led to the absurd conclusion that there could be no possibility of the basis for all scientific inference, induction. Dr. Stein, a scientist and professing atheist, had no answer to his own dilemma. His scientific endeavors were all based upon the borrowed capital of God’s providence. The absurd conclusion of “no induction” left Stein with the task of proving how induction was indeed possible without the God of providence whom Scripture has revealed. With attorney Edward Tabash, although Bahnsen argued briefly that induction presupposes God (with his “toothpaste proof for God’s existence”), I would say that Bahnsen’s emphasis was on the necessary preconditions for morality (given that Tabash had Jewish relatives who suffered under Hitler). With the negation of God's existence, Awschwitz was morally irrelevant. (Doug Wilson employed the same sort of reductio with Dan Barker at the University of Delaware about a decade ago.) In both cases, “the toothpaste proof for God’s existence” and the “morality” argument, Bahnsen demonstrated through MT that morality and induction have their only grounding in God.

In a word, arguing by reductio is the most powerful way of demonstrating that the very tools of argumentation presuppose that which the unbelieving worldview does not afford, God’s existence. That is done, as Bahsnen so often said, by “assuming for argument’s sake that God does not exist”, which is the very conclusion the professing atheist wants to lead us to in his argument. In summary then, the reason we first argue by negation and not affirmation is that negation allows us to assume the opponent’s conclusion. In other words, it allows us to better lead the opponent by the hand by starting with his presupposition in the minor premise.

Now, of course, if we apply MT as I do immediately below, in which case it is posited “no intelligible experience” in the consequent of the major premise, then we conclude with the existence of God (i.e. it is not the case that God does not exist). Accordingly, what was demonstrated before in Bahnsen’s coupling of the minor premise and the conclusion (i.e. no God, therefore, no intelligible experience) is now located in what the major premise posits. Such an argument is not as readily accessible in my estimation - first because of the two negatives that are used in the major premise, plus the double negative found in the conclusion. Notwithstanding, I have employed that very argument on this site when I was not so much interested in concluding the ramifications of the reductio (no intelligible experience), but rather when my intention was to lead to the conclusion that God exists (by using MT).

Step 1 (A--> B): If God does not exist, then there is no intelligible experience (since God is the precondition of intelligibility)
Step 2 (~B): There is intelligible experience
Step 3 (~A): It is not the case that God does not exist

Let’s now see what happens if we utilize MP rather than MT given the same major premise.

Step 1 (A--> B): If God does not exist, then there is no intelligible experience
Step 2 (A): God does not exist
Step 3 (B): Therefore, there is no intelligible experience

Unlike the argument that concludes God’s existence, this argument leads to the same conclusion as Bahnsen’s use of MT, but by using MP instead. I find this terribly cumbersome, however, because in the major premise “no intelligible experience” is posited as the necessary condition for no God, rather than God being positively posited as the necessary precondition for intelligible experience: “If intelligible experience, then God” (because God is the necessary precondition for intelligible experience). Accordingly, in this argument MP needs some reworking to make the point more clearly that intelligible experience presupposes God (as its necessary condition). In other words, although the conclusion is the same as Bahnsen’s, the opposing worldview is not reduced to absurdity because a reductio form is not employed. Also, this argument gives way to the five negatives employed in the two premises and conclusion.

Finally, Frame finds no problem with the following use of MP.

If causality, then God
Causality
Therefore, God

The reason Frame’s use of MP is not as affective as Bahnsen’s use of MT is similar to a reason stated before. Although all the negatives are done away with in this formulation, Frame’s argument does not conclude with the absurdity that there is no causality (or science, or ethics, etc.), which is the very thing the unbeliever would otherwise be challenged to overcome. In other words, because Bahnsen’s use of MT concludes the negation of that which the unbeliever affirms, the unbeliever is more abruptly confronted with the challenge to prove how intelligible experience can possibly comport with the minor premise of no God. Also, MT no less than MP can be formulated with additional commentary to address necessary preconditions (as opposed to mere necessary conditions). Therefore, MT can offer both, the transcendental challenge and the reductio in one short, sharp, shock. Bahnsen wins outright – point, set, match. (Also, Frame until late did not seem to fully appreciate that an argument from presupposition requires that God be the precondition for both the denial and the affirmation of the intelligible experience under consideration. He now acknowledges the point.)

Finally, Bahnsen once wrote:

“A transcendental argument begins with any item of experience or belief whatsoever and proceeds, by critical analysis, to ask what conditions (or what other beliefs) would need to be true in order for that original experience or belief to make sense, be meaningful, or be intelligible to us. Now then, if we should go back and negate the statement of that original belief (or consider a contrary experience), the transcendental analysis (if originally cogent or sound) would nevertheless reach the very same conclusion.” (Bahnsen, Van Til’s Apologetic, 501-502.)

What Dr. Bahnsen meant was that whether we argue for or against morality (for instance) we arrive at the same transcendental conclusion, God exists. Accordingly, to make sense out of the belief that there is no morality (or that there is morality) one must presuppose God’s existence in that investigation. It should be obvious that Bahnsen did not commit this fallacy:

If morality, then God
No morality
Therefore, no God

Rather, he simply meant that whether one claims to believe or not to believe X, then God. The reason being, God is the necessary precondition for all beliefs, true of false.

Ron


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