 This dispute is central to various issues in metaphysics and the philosophy of mind. For example, nonreductive physicalists often say that mental properties are distinct from but nonetheless “nothing over and above” physical ones. Their reductivist opponents, however, clearly think that this is illegitimate. This can be seen in the charge that nonreductive physicalists face the exclusion problem—that they are unable to account for the causal efficacy of the mental without claiming that all of its effects are “double-caused.” (See the entry on  mental causation.)