  The idea of possibility being rooted in arbitrary recombinations of  the actual world, rearrangements of its objects and universals, is  intuitively appealing. Clearly, however, not just any such  recombination can count as a possible world. Some states of affairs  are intuitively impossible — [being an elephant,  e], where e is an individual electron, say — and  some pairs of states of affairs, while individually possible, are not compossible — the states of affairs [having 1kg  mass, a] and [having 2kg mass, a] for a  given object a, or, for a given mereological sum m of  simples, the states of affairs [being a baboon, m] and  [being a hoolock, m]. But nothing that has been said  rules out the existence of recombinations of the actual world — rearrangements of its objects and universals — that include  such states of affairs. Obviously, however, such recombinations  cannot be thought to represent genuinely possible worlds. Of course,  like the abstractionist, the combinatorialist could simply stipulate  as part of the definition that all legitimate recombinations must be  genuinely possible states of affairs of a certain sort,  genuinely possible recombinations. But this will not do.  For, like concretism, combinatorialism purports to be a  reductive account of modality, an account of possible worlds that does not depend ultimately on modal notions (see Armstrong 1989,   33).[51] 