One implication of this view of levels, combined with certain familiar assumptions about causal relations, is that there can be no causal relationships between items at different levels of mechanisms. There can be causal relationships between things of different sizes, and there can be causal relationships between things described in very different vocabularies; but (again, conjoined with certain assumptions about the temporal asymmetry of cause and effect and the independence of cause and effect) there cannot be causal relationships between the behavior of a mechanism and the activities of the parts that jointly constitute that behavior. Claims about interlevel causation, which are ubiquitous in the scientific literature, are best understood either as targeting a different sense of levels or, concerning levels of mechanisms, as expressing hybrid claims combining constitutive claims about the relationship between the behavior of the mechanism as a whole and the activities of its parts, and causal claims concerning relationships between things not related as part and whole (Craver and Bechtel 2007). For alternative interpretations of levels, see Fehr 2004; Leuridan 2011; Thalos 2013; Eronen 2013; 2015; Baumgartner and Gebhardter forthcoming; Romero forthcoming. For reflections on the metaphysical status of higher-level phenomena and higher-level causes, see Baumgartner 2010; Glennan 2010 a, b; Hoffman-Kolss 2014, as well as the entries on   causation and manipulability,  physicalism, and  scientific reduction.