  For Lewis possible worlds are maximal sums of spatiotemporally related objects. A sum of spatiotemporally related objects is maximal if and only if nothing that is not part of the sum is spatiotemporally related to any part of the sum in question. Since sums of spatiotemporally related objects are sums of concrete objects, and sums of concrete objects are concrete objects, Lewisian possible worlds are concrete   objects.[25] , [26] 