From W David Buschart's essay on Jaroslav Pelikan in Historians of the Christian Tradition comes this quote from A N Whitehead, apparently a favorite quote of Pelikan's:

When you are criticizing the philosophy of an epoch, do not cheifly direct your attentions to those intellectual positions which its exponents feel it necessary explicitly to defend. There will be some fundamental assumptions which adherents of al the variant systems within the epoch unconsciously presuppose. Such assumptions appear so obvious that people do not know what they are assuming because no other way of putting things has ever occurred to them. With these assumptions a certain limited number of types of philosophic systems are possible, and this group of systems constitutes the philosophy of an epoch.

This is well said, but still leaves quite open the question of how to identify these assuptions. And all questions are a bit more difficult for me at the moment since my AC gave out this afternoon.