[westoncb]

Isomorphism, yes, exactly—that's the basic principle of a map: it's a description that enables us to make predictions about the territory, because it's possible to put the two into systematic correspondence. However once you start considering the 'representational' features of the map, which aren't an essential part of the isomorphism, as equally real (because you've made the simplification: dealing with the map is the same as dealing with the territory—our minds are predisposed to do this in many cases)—then you're in trouble.