in the philosophy of mind, theories that hold that mind and body are not separate, distinct substances but are composed of the same sort of neutral “stuff.”

David Hume (Hume, David), an 18th-century Scottish skeptic, developed a theory of knowledge that led him to regard both minds and bodies as collections of “impressions” (“perceptions”), the primary data of experience. Bertrand Russell (Russell, Bertrand), a 20th-century British logician and philosopher, called the neutral entities “sensibilia” and argued that mind and matter are “logical constructions.” William James (James, William), the American pragmatist, held that the neutral primary stuff is not a series of atomistic perceptions but is a “booming, buzzing confusion” that he termed “pure experience,” with mind, or consciousness, and body as names of discernible functions within it.

Neutral-monist theories have been criticized as inadequate in their account of either mind or body. Hume himself said (A Treatise of Human Nature) that his concept of mind as a bundle of perceptions inadequately accounts for the identity and simplicity of the mind. Others have criticized the notion that physical bodies comprise some sort of primary experience as implicitly idealistic. Hence, the central problem for neutral monism is seen as that of specifying clearly the nature of the neutral stuff without qualifying it in an exclusively mental or physical fashion.