The Presuppositionalist argues that everyone must presuppose the existence of God, in order to make the world intelligible. Thus ethics, logic, metaphysics, epistemology all presuppose the existence of God, more specifically, the Triune Christian God. To presuppose any x is to assume the truth of x by it's affirmation or it's denial (It's different then any x entailing y where x assumes the truth of y in it's affirmation but not in denial). So according to the Presuppositionalist when one denies the truth of Christianity one does so only by affirming the truth of Christianity! So we can construct the argument like this:
1. The world is intelligible
2. The world is only intelligible, if Christian Theism is true.
3. Therefore, Christian Theism is true.
Since everyone thinks the world is intelligibility entails Christian Theism. Notice however that in denying Christian Theism one also asserts the intelligibility of the world (that we can know at least some things, there are some objective facts, and so on), thus according to the Pressupositionalist we must assume the truth of this argument.
This argument is logically valid modus ponens, but it hardly appears sound. Premise 1 is clearly true, but why think premise 2 is true? The statement (a) Christian Theism is false and (b) The world is intelligible are not in any way saliently contradictory. So premise 2 is obviously needed in order for the claim that they (a) and (b) are inconsistent. There is as far as I can tell there is one central reason for supposing premise two is true. Namely, Christian Theism is the best way to make sense of the fact that the world is intelligible; because logic, ethics, and so forth all demand a foundation which can only be supplied via Christian Theism. This argument I will claim is entirely unconvincing. By doing so I will confront the issue of logic in isolation. I take it that if one need not presuppose the existence of God in order to affirm the truths of logic, one shouldn't have to presuppose the same for ethics, epistemology, metaphysics, and so on. Though of course the existence of God may best explain, be confirmed by, or entail such phenomena such arguments are noticeably different from the one advanced here.
Of course, it would be greatly appreciated if readers comment on ways in which the presuppositionalist argument could be better formed, or other arguments for premise 2--if there are such possibilities.
What We Actually Presuppose
Since the presuppositionalist demands that logic needs a foundation we might ask why. I suppose that they would say something like this: "You can only justify the use of reason with reason. That is begging the question. We must all beg the question this way, but the only view which makes sense of this is Christian Theism" or they might create a dilemma "Logic is either founded in physical things, social conventions, or the Christian God. Since it isn't physical and they aren't social conventions, Logic's foundation must be the Christian God." Lets take the latter argument first (Which is sounding a bit like the argument from reason, not really a transcendental argument). First, it doesn't seem entirely implausible (though improbable) that facts about logic are really physical facts, however lets grant the point that facts about logic cannot be founded in physical facts. Even so, there is no reason to grant Christian Theism. For Logic might be founded in necessarily existing abstract entities. Thus the state of affairs in which logic obtains occurs necessarily (it could not not occur). There is no possible world in which these state of affairs do not obtain. Hence we can affirm logic, while consistently denying the existence of God. After all this is what theists will say about the existence of God, that her existence occurs necessarily, that she cannot not exist and so on. But at this point the presuppositionalist may yell "no so fast!" and revert to the first criticism. This criticism has two problems, first we should be skeptical of the claim "We must all beg the question this way, but the only view which makes sense of this is Christian Theism." Why think this is true? Why think Christian Theism is the view that best makes sense of Logic? This question will be investigated below. Further though the statement "You can only justify the use of reason with reason. That is begging the question" reveals our actual presupposition, and I think the key for undoing the presuppositionalist argument. For look at these claims:
(4) because of logic you can't beg the question
(5) logic is justified because of logic and
(6) logic is logically unfounded because it begs the question.
First off (5) may very well be false, logic may be justified because it occurs necessarily void of any logical considerations, but lets grant that there is no escape route here. What do (4)--(6) assume? The truths of Logic! Hence Logic is presupposed in affirmation or denial. Thus it is a fundamental presupposition. To make this clear let's investigate this further by introducing a standard question begging argument.
(7) Christian Theism is true if the Bible is true
(8) The Bible is True
(9)Therefore, Christian Theism is true
If you don't like this example pick another one. Essentially the conclusion of this argument is assumed in one of the premises (since at least in this example, the claims Christian Theism is true and the Bible is true are equivalent). This goes wrong because of another thing which is assumed namely logic. We sketch out this arguments invalidity because of logic, and nothing else. Observe (Christian Theism is true=C, Bible is true=B, and Logic obtains=L); The argument for B and C is invalid because L. Compare this statement with the argument which claims logic needs support because it begs the question: The argument for L is invalid because of L. The only way this argument could work would be if the reasons for supporting logic beg the question, but in order for an argument to beg the question logic must obtain. The denial of L assumes the truth of L! Thus one asserts the truth of Logic by assertion and denial.
Yes, of course the presuppositionalist might reply, but logic still presupposes God. But how can one presupposition presuppose another? For it's not that logic entails God, it's that logic presupposes God. This seems incoherent, even if it isn't how could we know? For since the truth of logic itself is presupposed in discourse, how could it be dependent on God's existence? If God didn't exist could we couldn't say logic wouldn't exist, because we must presuppose logic! Thus we have a problem of incoherence, and epistemology. This objection appears to damn the presuppositionalist's argument.
Why Presuppose Christian Theism?
Even if what I said earlier is false there seems to be little reason to suppose we ought presuppose Christian Theism over other alternatives. Why not Islam, Mormonism, or Buddhism? Or better why not presuppose "the ultimate foundation being" which is an impersonal, amoral being which is the foundation of all things. Or perhaps, a set of beings or entities which together found logic? Christian Theism hardly seems to have the upper hand against these alternatives, and due to atheological arguments it may very well have a lower one. Surely we ought to have reasons for preferring Christian Theism over what could very well be an infinite number of alternatives capable of grounding the intelligibility of the universe.
On the Fear of Pleasing the Unbeliever
As such I am inclined to pronounce the presuppositional apologetic strategy, as formulated, a complete failure. In closing I would like to ponder whether the strategy is reasonably motivated by theology. Here is a quote from presuppositionalist John Frame in response to the charge that presuppositionalism begs the question:
"God created our minds to think within the Christian circle: hearing God’s Word obediently and interpreting our experience by means of that Word. That is the only legitimate way to think, and we cannot abandon it to please the unbeliever. A good psychologist will not abandon reality as he perceives it to communicate with a delusional patient; so must it be with apologists."
The argument here is that using neutral standards such as reason and evidence (Natural Theology, the approaches of Swinburne, William Lane Craig etc.) surrenders the ground to the unbeliever whose whole standards of reason and evidence depends on the presupposition of the Christian God. But as we saw earlier there is no need to do such a thing, further what we presuppose are standards of evidence and reason! So we (the apologist, philosopher, naturalist, skeptic, whatever) are like psychologists who argue with other psychologists by accepting common standards of reason and evidence, not like the some psychologists attempting to communicate with some deluded patient. As such this fear of pleasing the unbeliever is unfounded, for despite Frame's words, his life testifies to the fact that he is already pleasing many unbelievers. Thus it would seem as though Presuppositional Apologetics is not only a philosophical failure, but a theological one as well.