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Modal Realism/Stalnaker: (thesis that there are possible worlds) - Vsmodal realism: objection: it is not possible, to know any metaphysical facts about it - (whether possible world exist) - thesis: there is no strategy to counter this objection that would be analog to VsBenacerraf.
Benacerraf: tension between the need for a plausible representation of mathematical statements and the representation of our respective knowledge about their truth.
Platonism: gives plausible semantics but no epistemology. - Reference/Benacerraf: thesis: needs causal link. - LewisVsBenacerraf: does not apply to abstract objects such as numbers and so on.
Conclusion: we cannot distinguish Platonism in terms of mathematical objects from that in terms of possible worlds.
Modal Realism/VsMR/Possible world/Stalnaker: Problem: the MR cannot say on the one hand that possible worlds are things of the same kind as the real world (contingent physical objects) and on the other hand, that possible worlds are things of which we know in the same way as of numbers, etc. - MR: will insist on the fact that even the reference to ordinary objects (actual or merely possible) needs no causal connection.
Ways a World may be Oxford New York 2003