 In order to avoid unlimited pancomputationalism, (discussed in Section 3), Chalmers (1995, 1996, 2011) adds the further restriction that a physical implementation of a computational system must divide into separate physical components, each of which maps onto the components specified by the computational formalism. As Godfrey-Smith (2009, 293) notes, this combination of a causal and a localizational constraint goes in the direction of mechanistic explanation (Machamer, Darden, and Craver 2000). An account of computation that is explicitly based on mechanistic explanation will be discussed in Section 2.5. For now, the causal account requires only that the mappings between computational and physical descriptions be such that the causal relations between the physical states are isomorphic to the state transitions specified by the computational description. Thus, according to the causal account, concrete computation is the causal structure of a physical process.