It is the actual existence of the God of Christian theism and the infallible authority of the Scripture which speaks to sinners of this God that must be taken as the presupposition of the intelligibility of any fact in the world. (Van Til p. 139)
Scripture presents itself as being the only light in terms of which the truth about facts and their relations can be discovered. ( Ibid.p. 130)
[T]he believer must defend God’s word as the ultimate starting point, the unquestionable authority, the self-attesting foundation of all thought and commitment. (Bahnsen p. 74)
The presuppositionalist claim here seems quite clear. The Christian scriptures (i.e., the word of God) must be taken as the foundation and starting point of our reasoning. That is to say, the Bible must be presupposed in order for any facts to be made intelligible. Hence, no valid conclusions or inferences can be made without utilizing scripture as our epistemological foundation.
Immediately we run into problems when this line of reasoning is promulgated. First, what constitutes scripture is not at all self-evident. In fact, some different Christian denominations have different canons and therefore different scriptures. (e.g., The Catholic canon is larger, and hence different, than the protestant canon.) The relevance that this has for presuppositionalism is that determining the constitution of scripture is itself an inference. Now, a conclusion or inference necessarily is justified by prior premises or propositions. Thus, how can scripture be a presupposition—that is, a foundation of thought—if it relies on prior premises to justify it? Obviously it can’t. The nature of a presupposition is that it comes first and foremost at the beginning of our epistemology. A presupposition ultimately provides justification for every other subsequent inference. So, scripture cannot even possibly be presupposed since scripture itself relies on premises that precede it in order to justify its constitution.
Second, if facts can only be made intelligible by making scripture a presupposition, then it necessarily follows that any inferences or conclusions reached without this presupposition are unintelligible. But again, the constitution of scripture can only be reached through prior premises and inferences. And obviously these prior premises did not have scripture as a presupposition, since they precede and lead up to the conclusion of what constitutes scripture. Now, since these premises did not presuppose scripture, then any conclusions they reach are unintelligible—by the presuppositionalist’s own claim. Hence, the conclusion of what constitutes scripture must be itself unintelligible . Subsequently, if presuppositionalism were true then the conclusion of what constitutes scripture would be unintelligible and therefore scripture itself would be unintelligible; and thus we could never presuppose it.What we’ve witnessed here is that once again the presuppositionalist does not understand the nature of epistemology. The very thing they want to presuppose cannot in fact be presupposed, since it already relies on prior premises for its justification. And since this betrays the nature of presuppositions themselves then the presuppositionalist only pulls the rug out from underneath them.