  Frith’s idea is that a causal explanation of a psychotic phenomenon, or some other mental symptom, must cite properly cognitive variables to be explanatory. That is, explanations should relate variables from the same level. The relationship between dopamine and thought is just opaque to reason, and we want variables that are related in ways that are transparent to reason. There is a constraint here; the idea that explanations of psychological phenomena should relate cause and effect in ways that we find intelligible. Campbell (2008) in attacking this picture, notes that talk of levels of explanation is often wheeled in at this point, as it is by Frith, to supply relations that are transparent to reason in the right way. The idea is that only processes at a level let us grasp explananda at that level. Explanantia at a different level are not intelligible in the right way, so although lower-level processes may realise the higher ones they cannot explain them. This amounts to a constraint on acceptable explanation. Should we accept it?