Taking this latter point one step further, Lewis argues that modality cannot be made sense of "without" such a reduction. He maintains that we cannot determine that "x" is possible without a conception of what a real world where "x" holds would look like. In deciding whether it is possible for basketballs to be inside of atoms we do not simply make a linguistic determination of whether the proposition is grammatically coherent, we actually think about whether a real world would be able to sustain such a state of affairs. Thus we require a brand of modal realism if we are to use modality at all.