  What of existence in such worlds? As we've seen, on Lewis's account,  to exist in a concrete world w is literally to exist in  w, that is, within the spatiotemporal boundaries of  w. Clearly, because SOAs are abstract, individuals cannot  exist in abstractionist worlds in anything like the same literal,  mereological sense. Accordingly, the abstractionist defines existence in a world simply to be a special case of the inclusion relation: 