 A third candidate for physical quiddities, which is similar to Lockean solidity, may be found in contemporary metaphysics. Many contemporary metaphysicians accept that there exist non-Humean causal powers (see the entry  dispositions).  Humeans identify causal powers with mere tendencies, which can be fully explicated by means of subjunctive conditionals such as, “If an electron were in the vicinity of a proton, it would attract that proton” (Jacobs 2011). By contrast, non-Humeans identify causal powers with properties that categorically ground tendencies—much as Lockean solidity grounds impenetrability. Such categorical properties are truthmakers for, and not explicable in terms of, subjunctive conditionals. For example, the causal power of an electron is a categorical property that makes subjunctive conditionals such as the one just specified (“If an electron…”) true (Jacobs 2011; cf. Heil 2003, Jaworski 2016). Such truthmakers could be construed as physical quiddities (Gundersen 2015, Pereboom 2016).