Or, rather, the area of philosophy where only the class of facts which can be certainly described as such are the facts of having particular present sensations (whether the kind that present as sense-data or the kind that present as memories) is a fact, and all other ideas about the world are either axioms or conclusions supported on the basis of either axioms, sensations, or a combination of the two.

Of course, when you leave that area of philosophy, you are in the area of fuzzy and imprecise thinking, so its not a bad area of philosophy to be in.