Davidson uses the interaction principle to establish directly one part of mental anomalism—psychological anomalism, which denies the possibility of strict, purely psychological laws of the form ‘M1 & M2 → M3’ (Davidson 1970, 224; 1974b, 243). For if physical events causally impact on mental events, then the mental domain is ‘open’, and any laws in which mental predicates figure will have to take this into account (for related discussion, see the supplement on Causal Closure of the Physical