 The first basic posit of the structuralist theory of representation is that models are structures in this sense (the second is that models represent their targets by being suitably morphic to them; we discuss morphisms in the next subsection). Suppes has articulated this stance clearly when he declared that “the meaning of the concept of model is the same in mathematics and the empirical sciences” (1960 [1969]: 12), and many have followed suit. So we are presented with a clear answer to the Problem of Ontology: models are structures. The remaining issue is what structures themselves are. Are they Platonic entities, equivalence classes, modal constructs, or yet something else? In the context of a discussion of scientific representation one can push these questions off to the philosophy of mathematics (see the SEP entries on the  philosophy of mathematics,  nominalism in the philosophy of mathematics, and  Platonism in the philosophy of mathematics  for further details).