  The essence of van Fraassen's objection here is that the difference between mathematical (uninstantiated/abstract) structure, and physical (instantiated/concrete) structure cannot itself be explained in purely structural terms. There is an analogy here with the theory of universals and the problem of exemplification. A similar complaint is made by Cao (2003a and 2003b). Esfeld (2013) uses this objection in the context of the interpretation of quantum mechanics to pose a dilemma for ontic structural realism.