 Isomorphism is more restrictive than similarity: not everything is isomorphic to everything else. But isomorphism is still too abundant to correctly identify what a model represents. The root of the difficulties is that the same structures can be instantiated in different kinds of target systems. Certain geometrical structures are instantiated by many different systems; just think about how many spherical things we find in the world. The \(1/r^2\) law of Newtonian gravity is also the “mathematical skeleton” of Coulomb’s law of electrostatic attraction and the weakening of sound or light as a function of the distance to the source. The mathematical structure of the pendulum is also the structure of an electric circuit with a condenser and a solenoid (Kroes 1989). The same structure can be exhibited by more than one kind of target system, and so isomorphism by itself is too weak to identify a model’s target.