 I should forestall one possible misunderstanding. You might think that, given what I’ve just said, the way Lewis would recommend doing philosophy is as follows: First you figure out what your basic metaphysical commitments should be; then you turn your attention to various broad but non-foundational philosophical subject matters (personal identity, mental content, the nature of knowledge, theory of value, etc.), and work out the consequences in each of these arenas of your fundamental metaphysical posits. Nothing could be further from Lewis’s preferred methodology. (Well, maybe relying on divine revelation would be further….) What he in fact recommends is a holistic approach: we start with the total body of claims we are inclined to believe—whether on the basis of “common sense” (an oft-invoked category, for Lewis) or of science—and try our best to systematize it in accordance with standards of theoretical goodness that are themselves endorsed by common sense and/or science (and so are themselves, to some extent, also up for grabs). A substantial portion of Lewis’s overall body of philosophical work can thus be seen as an extended—and breathtakingly ambitious—attempt at achieving total reflective equilibrium. Here is an especially succinct description of this approach: