 (PW) makes use of modal concepts to define possible world: it must be possible for S and w to both obtain etc. Hence, this particular Abstractionist conception of possible worlds does not allow one to explain modality away; it presupposes it. In contrast, the Concretist conception promises a definitional reduction of modal concepts. When we talk about possibility and necessity we quantify over mereological sums of things. The concept of modality is not required to distinguish between possible and impossible worlds: there are no impossible ones. Why is (PW), then, attractive?