Neither Mental nor Physical, but the Source of Both

Bertrand Russell set out neutral monism most carefully in works from the 1920s, arguing that what we call a physical object and what we call a perception of it are actually built from the same raw material, momentary sensory events he called sensibilia, organized one way when considered as part of a causal physical order and another way when considered as part of a perceiving mind's biography. On this view, asking whether a given sensory event is really mental or really physical is a bit like asking whether a given point in space is really horizontal or really vertical, a question that only makes sense once a frame of reference has been chosen, not a question with an answer built into the point itself.

William James's radical empiricism anticipated the same move: for James, pure experience is prior to the mind-matter distinction, and both terms of that distinction are later, useful ways of sorting one and the same primal stuff. The position's chief technical burden is explaining, in detail, how organizing neutral elements one way yields a full physical causal order and organizing them another way yields a subject with a genuine point of view, a burden critics argue neutral monists have never fully discharged, leaving the position more a promissory note than a worked out theory.