 In the late twentieth century, Grover Maxwell (1978) and Michael Lockwood (1989, 1992) each endorse a theory that they attribute to Russell and is often interpreted as Russellian monism (see, for example, Chalmers 1996, 2013). Maxwell presents his theory in the course of defending the mind-brain identity theory (Place 1956; Smart 1959; Lewis 1966; see the entry  the mind/brain identity theory)  against Saul Kripke’s (1972) influential challenge. Kripke’s challenge concerns how to reconcile that identity theory with the “apparent contingency of the connection between the mental state and the corresponding brain state…” (Kripke 1972 [1980: 152]). Maxwell rejects an assumption on which, he claims, that challenge relies: the assumption that “we know from common sense, from physics, from neurophysiology, etc., what brain events are like” (Maxwell 1978: 134). In his view, those sources tell us about structural properties but not about brain events’ underlying non-structural properties—a line of reasoning suggestive of both structuralism about physics and realism about quiddities. He also seems to endorse something like quidditism about consciousness. In the case of brain events that are, in his view, identical to macrophenomenal experiences, he implies that their underlying non-structural properties might be phenomenal properties—a claim suggestive of Russellian panpsychism. Lockwood (1989, 1992) develops a theory similar to Maxwell’s, remarking that it “seems to me the only approach to the philosophical mind–body problem, currently on offer, that holds out the slightest promise” (Lockwood 1992 [2015: 145]).