 According to the simple mapping account, a physical system S performs a computation defined by description C just in case (i) there is a mapping from the states ascribed to S by a physical description to the states defined by computational description C, such that (ii) the state transitions between the physical states mirror the state transitions between the computational states. Thus, suppose S has states p1 and p2 that map on states s1 and s2 of C (respectively). Clause (ii) requires that, for any computational state transition of the form s1 → s2, S transitions from p1 to p2 whenever it goes into state s1.