The ontological problem—the problem of the acceptability of the ontological commitments made by the mathematical fictionalist—is basically solved. No commitment to mathematical objects is, in principle, made. Although a primitive modal notion is introduced, it has only a limited role in the nominalization of mathematics: to allow for a nominalist formulation of the crucial concept of conservativeness. As we saw, however, without a proper nominalization of set theory itself, it is unclear whether the mathematical fictionalist program ultimately succeeds.