  The question about the nature of possible worlds is a hotly debated topic. Some, for instance Alvin Plantinga, think that possible worlds are states of affairs that are both possible and maximal. A maximal state of affairs is one that includes or precludes every state of affairs — where a state of affairs S includes a state of affairs S* if and only if it is not possible that S obtain and S* fail to obtain, and S precludes S* if and only if it is not possible that both obtain (Plantinga 1974, 45; 2003a, 107; 2003b,  194).[21]   According to Plantinga possible but not necessary states of affairs can obtain and can fail to obtain. Those states of affairs that obtain are actual. The actual world includes every actual state of affairs (Plantinga 2003a, 107; 2003b, 195). Merely possible states of affairs and worlds exist but do not obtain (Plantinga 2003a, 107; 2003b, 195). States of affairs, and therefore possible worlds, are thought of as abstract objects by Plantinga. Indeed even the actual world is an abstract object for Plantinga, since it has no center of mass, it is neither a concrete object nor a mereological sum of concrete objects and, like the state of affairs of Ford's being ingenious, has no spatial parts at all (2003a, 107).