 Similar in some ways to Couch’s arguments (discussed in  §2.4  above), Colin Klein raises a challenge to the scientific contributions made by multiply realized kinds. Noting the variety of things that materials science classifies as “brittle,” Klein (2008) notes that few to none of the many scientific discoveries about realization-restricted brittle things—for example, about brittle steel—generalize to other realization-restricted types—such as brittle glass. Klein insists that generalizations about genuine scientific kinds should be projectable across instances of those kinds, so this requirement seems not to be met by a significant class of multiple realized kinds, namely, the realization-restricted ones. Applying this point to psychological kinds, instead of supporting a scientifically-backed nonreductive physicalism, it appears rather that special sciences should abandon multiply realized kinds. Klein notes that proponents of scientifically-based multiple realizability can find terms in special sciences that figure in legitimate explanations, and so appear to refer to projectable multiply realize kinds. But close investigation of some paradigmatic examples reveals these to be idealizations of actual kinds. Special-science kind-terms are thus typically ambiguous. Sometimes a given term refers to an actual but realization-restricted kind. Other times it refers to features of explanatory but non-actual idealized models. (Klein 2008 illustrates this ambiguity with his detailed example from materials science.) Neither suffices to provide a kind of actual multiple realization that the standard argument requires. However, he insists that his argument isn’t entirely negative for non-reductive physicalism. Idealizations can function in explanations that are autonomous in an important sense from lower level sciences. And furthermore, Klein argues, Jaegwon Kim’s (1992) assumption that all explanatory work in science must appeal to realization-restricted kinds and properties (discussed in  §2  above), is simply incorrect. Still, Klein insists, there appear to be no actual and projectable—hence genuinely scientific—multiple realized kinds.