Quoting from Greg Bahsen’s Always Ready: Directions for Defending the Faith, Chapter 26, Answers to Apologetic Challenges:
If a dispute arises over the price of eggs at the store, we can jump in the car, drive down to the market, and go look for ourselves at the price listed on the eggs. If scientists disagree over the claim that smoking causes cancer, they can run tests, do statistical comparisons, etc. In such cases, it seems that what we do, at base, is “look and see’ if one hypothesis or its opposite is true. Of course, disagreements such as these can be readily resolved in this fashion only because the two people who disagree nevertheless agree with each other regarding more basic assumptions—such as the reliability of their senses, the uniformity of natural events, the accuracy of data reporting, the honesty of researchers, etc.
However, when the dispute is over more fundamental issues, as it is between believers and unbelievers, simple appeals to observational evidence need not be decisive at all. The reason is that a person’s most fundamental beliefs (or presuppositions) determine what he or she will accept as evidence and determine how that evidence will be interpreted. Let me illustrate. Naturalism and supernaturalism are conflicting outward looks regarding the world in which we live and man’s knowledge of it. The naturalist claims that what is studied by empirical science is all that there is to reality, and that every event can (in principle) be explained without resorting to forces outside the scope of man’s experience or outside the universe. Christian supernaturalism, on the other hand, believes that there is a transcendent and all-powerful God who can intervene in the universe and perform miracles which cannot be explained by the ordinary principle of man’s natural experience. Now then, having well-accredited reports of a “miraculous” event is not in itself sufficient to change the mind of the naturalist—and for good reason. The naturalist’s presuppositions will require him to dispute the claim that such an event really occured, or alternatively, will lead him to say that the event is subject to a natural explanation once we learn more about it. Simple evidence need not dislodge his naturalistic approach to all things—any more than simple eye-ball evidence could ever in itself refute the Hindu conviction that everything about man’s temporal experience is Maya (illusion). Our presuppositions about the nature of reality and knowledge will control what we accept as evidence and how we view it.
I’m pretty sure, if you’ve had any apologetic discussions—if you’ve ever tried to explain why you believe as you do—with someone committed to another worldview, you’ve seen this kind of presuppositional disconnect come up one way or another.
Back in my early days of blogging, a commenter who called himself Homopope (You’ll see him quoted on the sidebar.) wanted to argue about the existence of God. “If God exists,” he asked, “why doesn’t he just call me up on the phone and tell me so?” So I asked back, “If God called you up and said, ‘Hello, this is God and I exist,’ would you believe him?” The answer (Do you see it coming?) was that he’d know the caller was someone other than God, because (Wait for it!) God doesn’t exist. Yup, he started with the presupposition that there was no God.
There was a similar disconnect in the discussion with Godlessons (start there and work backward if you must) last fall. Godlessons started with the presupposition that we can only know things that can be proved empirically.