 There remains a final problem to be addressed in the context of structural accounts of scientific representation. Target systems are physical objects: atoms, planets, populations of rabbits, economic agents, etc. Isomorphism is a relation that holds between two structures and claiming that a set theoretic structure is isomorphic to a piece of the physical world is prima facie a category mistake. By definition, a morphism can only hold between two structures. If we are to make sense of the claim that the model is isomorphic to its target we have to assume that the target somehow exhibits a certain structure \(\mathcal{S}_T\). But what does it mean for a target system—a part of the physical world—to possess a structure, and where in the target system is the structure located?