Lewis believed that all knowledge about the world, even simple perceptual truths, is hypothetical, taking the form of “If I do X, then Y will result.” For example, to say that the wall is hard, means that I will have a certain sensation if I bang my head against it, just as the claim that the peach is ripe means that if I bite into it, I will experience certain expected flavors.

In ethics, Lewis believed that value judgments are appraisals of the consequences of action. Aesthetic valuation, however, involves an apprehension of an objective qualitative mode of experience. Lewis, like John Dewey (1859–1952), believed that values are in the world, as objective qualities, and not the result of human preferences or judgments. According to Lewis, every experience has both a value dimension, according to where it is on a scale from good to bad, and an aesthetic dimension from pleasing to unpleasant, or of high to low aesthetic quality. In both ethics and aesthetics, some things can be seen to be intrinsically good, upon reflection. And in ethics, the aim and purpose of action is often what is intrinsically good.