The ontological problem consists in specifying the nature of the objects a philosophical conception of mathematics is ontologically committed to. Can the nature of these objects be properly determined? Are the objects in question such that we simply lack good grounds to believe in their existence? Traditional forms of platonism have been criticized for failing to offer an adequate solution to this problem. In response, some platonists have argued that the commitment to mathematical objects is neither problematic nor mysterious (see, e.g., Hale and Wright 2001). Similarly, even though some nominalists need not be committed to mathematical objects, they may be committed to other entities that may also raise ontological concerns (such as possibilia). The ontological problem is then the problem of assessing the status of the ultimate commitments of the view.