 On the one hand, from the perspective of empirical science, co-intensionality may appear too strong as a criterion of identity. For the identity statements of scientific reductions, such as that of temperature to mean kinetic energy, could suggest that some properties are identical even if not co-intensional. For example, one may accept that having absolute temperature of 300K is having mean molecular kinetic energy of \(6.21 \times 10^{-21}\) (Achinstein 1974: 259; Putnam (1970: §1) and Causey (1972) speak of “synthetic identity” and “contingent identity”, respectively). Rather than logical necessity, it is nomological necessity, necessity on the basis of the causal laws of nature, that becomes central in this line of thought. Following it, some have focused on the causal and nomological roles of properties, i.e., roughly, the causes and effects of their being instantiated, and their involvement in laws of nature, respectively. They have thus advanced causal or nomic criteria. According to them, two properties are identical iff they have the same causal (Achinstein 1974: §XI; Shoemaker 1980) or nomological role (Swoyer 1982; Kistler 2002). This line has been influential, as it connects to the “pure dispositionalism” discussed in  §5.2.  There is however a suspicion of circularity here, since causal and nomological roles may be viewed as higher-order properties (see entry on  dispositions, §3).