 A related worry is that (1)–(4) don’t say anything about the nature of neutral entities. (1) says only what they are not; (2)–(4) specify certain roles that neutral entities can play, without telling us anything about the nature of the role occupants. (5) may fare better in this regard; but it raises problems of its own (see below). Some versions of neutral monism embrace this agnosticism about the nature of the neutral base. David Chalmers has outlined the idea of Russellian neutral monism (see section  5.5  below) with “fundamental protophenomenal properties serving as the grounds for both physical and phenomenal properties” (Chalmers 2015: 264). Nothing is known about the protophenomenal other than its ability to ground the two kinds of properties we are familiar with.