“Presuppositions are the deciding factor in determining how facts are interpreted and combined to give particular content to a worldview. “A presupposition is something assumed or supposed in advance…One could say that to ‘presuppose’ is to conclude something before the investigation is commenced.” A presupposition is not proved by anything else more ultimate. For purposes of reaching answers to fundamental questions, a presupposition is “a belief over which no other takes precedence.”
A person who asserts that all things must be tested by the standard of reason has assumed-ultimately presupposed- that reason is the test for truth. If he appeals to some other princuple to verify that reason is ultimate, then that new principle becomes ultimate. When there is no other principle upon which to appeal, an ultimate presupposition has been established. Ultimately, how does a person know that reason- or anything else, for that matter- is the standard? Greg L. Bahnsen describes it this way:
All argumentation about ultimate issues eventually comes to rest at the level of the disputant’s presuppositions. If a man has come to the conclusion, and is committed to the truth of a certain view, P, when he is challenged as to P, he will offer supporting argumentation for it, Q and R. But of course, as his opponent will be quick to point out, this simply shifts the argument to Q and R. Why accept them? The proponent of P is now called upon to offer S, T, U, and V as arguments for Q and R. And on and on the process goes. The process is complicated by the fact that both the believer and unbeliever will be involved in such chains of argumentation. But all argument chains must come to an end somewhere. One’s conclusions could never be demonstrated if they were dependent upon an infinite regress of argumentative justifications, for under those circumstances the demonstration could never be completed. And an incomplete demosntration demonstrates nothing at all.
Eventually all argumentation terminates in some logically primitive starting point, a view or premise held as unquestionable. Apologetics traces back to such ultimate starting points or presuppositions. In the nature of the case these presuppositions are held to be self-evidencing: they are the ultimate authority of one’s view point, an authority for which no greater authorizations can be given.
(DeMar, Thinking Straight in a Crooked World, 109-110)