  An SOA is said to be possible (necessary,  impossible) insofar as it is possible (necessary, impossible)  that it obtain. One SOA s is said to include another  t if, necessarily, s obtains only if t does;  s precludes t if, necessarily, s obtains  only if t doesn't. So, for example, Algol's being John's  pet includes Algol's being someone's pet and precludes  there being no pets. Thus, on the abstractionist's  understanding of a situation as a state or condition of the physical  world rather than a concrete, structured piece of it, the inclusion  of one situation in another is a purely logical relation, not  a   mereological  one. Finally, say that an SOA s is total if, for every  SOA t, s either includes or precludes t.  (Abstractionists often use ‘maximal’ instead of  ‘total’, but we have already introduced this term in the  context of concretism.) Abstractionist possible worlds are now  definable straightaway: 