  The combinatorialist could of course abandon actualism and include  merely possible simples into her ontology. Again, she could follow  the new actualists and draw a division between actually concrete and  non-actual, possibly concrete simples; or she could introduce  Plantinga-style haecceities to go proxy for merely possible simples.  But all of these options would be badly out of step with the strong,  naturalist motivations for combinatorialism: There is but the one  physical world comprising all of the facts; recombinations of (at  least some of) those facts — arbitrary rearrangements of  their simple objects and universals — determine the possible  worlds. Mere possibilia, merely possible non-concretia, and non-qualitative haecceities have no real place in that picture. 