 So, there are at least some candidates for what physical quiddities might be like (for others, see, e.g., Chalmers 2013 [esp. 2015: 270–74]; Rosenberg 2004; Coleman 2012, 2015). Still, one might wonder if any of the candidates just described—Aristotelian prime materiality, Lockean solidity, or non-Humean causal powers—can serve all of the Russellian physicalist’s purposes. Russellian physicalists argue that positing physical quiddities safeguards their theory from the conceivability and knowledge arguments  (section 3.1).  But suppose those quiddities are construed as non-Humean causal powers of physical entities such as electrons. Wouldn’t Mary be in a position to fully understand such powers while she is still in the room? (Pereboom 2011: 113–14). If so, then the proposed construal would seem to undermine the Russellian physicalist’s basis for denying that her pre-release physical knowledge is incomplete. Similarly, one might argue that the proposed construal would undermine the Russellian physicalist’s contention that while a structural zombie world is ideally conceivable, a full-fledged zombie world is not. Parallel concerns arise for Aristotelian prime materiality and Lockean solidity. Even if those properties could be construed as physical quiddities, they would not appear to facilitate the Russellian physicalist’s objective of providing a plausible response to the conceivability and knowledge arguments.