 Russellian monism is a distinctive theory in the metaphysics of mind. Nevertheless, it is possible to identify varieties that correspond to more traditional theories, including physicalism, idealism, neutral monism, and (despite the name) even dualism (see the entries  physicalism;  idealism;  neutral monism;  and  dualism).  On physicalist Russellian  monism, both quiddities and structural properties are physical  (Stoljar 2001; Pereboom 2011, 2014, 2015; Coleman 2012, 2015; Montero  2015; Morris 2016; Brown 2017). On idealist Russellian monism, both  quiddities and structural properties are mental (Bolender, 2001;  Adams 2007; Schneider 2018; cf. Chalmers 1996: 155). On neutral  monist Russellian monism, both quiddities and structural properties  are neutral, where neutral properties are neither physical nor mental  but underlie both physical and mental properties (Nagel 1986, 1998;  cf. Stoljar 2015; Wishon 2016; also see the discussion  of panqualityism in the entry  panpsychism). Finally, Russellian monism could be combined with the traditional (e.g., Cartesian) dualist view that the concrete world includes two fundamentally distinct sorts of entities, the physical sort and the mental sort. This can be done by, for example, identifying structural properties with physical properties and quiddities with certain mental properties (cf. Chalmers 2010: 135).