 First, there is the “incompleteness problem” discussed by Rosen in Rosen 1990, pp. 341–345. There are some modal issues (and corresponding issues about the nature of possible worlds) that a realist may well be silent on: not because they believe there is no answer, but rather because they believe themselves ignorant of the answer. A fictionalist who treats the realist’s theory as a fiction, on the other hand, will be silent upon the same issues — but this can lead to a more serious problem. If the fiction is silent on an issue (Rosen’s example concerns the size of worlds), it is not that the issue is unknown — it is that the fiction does not represent a fact of the matter one way or the other. So it might appear for the fictionalist there is not an unknown modal fact — either the claim is false because the corresponding worlds-claim is not true according to the fiction, or something involving a truth-value gap is going on. Rosen also discusses what effect this might have on modal claims corresponding to such silences. A detailed discussion and critique of Rosen on this issue can be found in the following supplementary document: