Thinking about my claims concerning possible worlds, I realized that my argument rested too much on a controversial claim: worlds do not have counterparts because counterparts are not maximally including sets of what there is. I still think this claim is true, but Sam made me doubt as to whether it is so evidently true.
I still think, however, that there is a big problem with possible worlds and, in particular, with modal realism. I'll frase it in the following way.
(P1)All propositions about reality, if true, are necessarily true.
(P2)Propositions about possible worlds are propositions about reality.
(P3)From P1 and P2, it follows that propositions about possible worlds are necessarily true when true.
(P4)From P3 it follows that there are no contingent truths about possible worlds.
(P5)From P4 it follows that all properties of worlds are essential to them, and thus, that worlds are necessarily the way they are.
However, claims about members of worlds (e.g. that I could possibly be a plumber), are claims about what there is and, of course, claims about possible worlds. That there is a world in which someone pretty much like me is a plumber is, if true, necessarily true.
(P6) From P5 it follows that all properties of individuals are essential properties.
If P6 is true then we can't make sense of accidental properties and neither of modal properties of objects. All there is to an object is the set of properties that it actually (or even instantly) has. Every single change is a change of identity, for it will imply a change of an essential property -and at the end there will just be no change at all.
It seems then that the reduction of modal concepts offered by modal realism amounts simply to the claim that there are no modal properties. As much as, I think, it implies that there are no essential properties.
