  Another, now less common, argument against Platonism, is that its ontology is unintelligible. Sometimes the unintelligibility of abstract objects is linked to their lack of clear and intelligible conditions of identity. But it is not the abstractness of abstract objects that makes them lack clear identity conditions, since some abstract objects, like sets, have clear and intelligible conditions of identity. But the identity conditions for sets are intelligible only if the notion of a set is intelligible. Some, like Goodman, are apparently unable to understand how different entities can be composed out of the same ultimate constituents. But, again, it is not in virtue of being abstract, i.e. non-spatiotemporal and causally inert, that sets violate Goodman's principle on composition. For there could be simple abstract objects.