 One way to construct more restrictive accounts of computation is to further constrain acceptable mappings. As a reminder, the simple mapping account has it that, for any computational state transition of the form s1 → s2, if the system is in the physical state p1 that maps onto s1, it then goes into the physical state p2 that maps onto s2. The second part of (ii) is a material conditional. This account may be strengthened by turning it into a conditional expressing a relation that supports counterfactuals.