Jeff at Dawn Treader posts a very nice summary of the different classes of presuppositions people might bring to the knowledge table, and asks whether there is a way to adjudicate among them. Here’s my take: not to pound a drum, but it seems to me that foundationalism breaks down for the very reason that it’s impossible to choose between competing foundational presuppositions.
One way to choose between competing presuppositions, as one of Jeff’s commenters suggested, might be by some criterion like falsifiability or testability. Even then, however, we have to presuppose that our criterion is meaningful and will lead to truth. A counter-example might suffice to falsify a claim, but only if the counter-example was observed and reported without significant bias, and there were no external factors affecting the example. You have to presuppose away observation and reporting bias as well as external factors. Presuppositions are built on presuppositions are built on presuppositions…. it never ends. Only God knows the absolute, total truth for sure, or, if there is no God, no one does.
For example, say we test the theory of gravity by dropping bowling balls from a roof. What if one bowling ball out of a hundred floats up rather than falling down, because of a miraculous intervention by God. If we presuppose naturalism, and eliminate any other naturalistic causes, we’ve falsified the theory of gravity. But we’d be wrong, because our presuppositions are wrong.
It seems to me this is where alternatives or adjuncts to foundationalism come in. A test of coherence, for example, can help. Which set of beliefs provides the most coherent account of human experience and history? We can ask, say, whether naturalism provides a more coherent account of altruism and the moral sense than Christian theism. If we combine the various tests available to us — reason, “common sense” perception, coherence — we can gain some anchors for the presupposition or faith commitments on which we choose to base our lives. At the end of the day, there’s always the possibility we could be wrong, but the hand we’re dealt as human beings is that we’re fallible and limited, so we have to make commitments based on probabilities using the tools available to us.