The non-reductive physicalist is committed to following three principles: the irreducibility of the mental to the physical, some version of mental-physical supervenience, and the causal efficaciousness of mental states. The problem, according to Kim, is that when these three commitments are combined with a few other well-accepted principles, an inconsistency is generated that entails the causal impotence of mental properties. The first principle, which most ontological physicalists would accept, is the causal closure of the physical domain, according to which, every physical effect has a sufficient physical cause. The second principle Kim notes is that of causal exclusion, which holds that no normal event can have more than one sufficient cause. The problem is that a behavior cannot have as its cause, both a physical event and a (supervening) mental event, without resulting in a case of overdetermination (thus violating the principle of causal exclusion). The result is that physical causes exclude mental states from causally contributing to the behavior.