APA Dictionary of Psychology

a position holding that there is a single substance to reality, but that this is neither physical (body) nor mental (mind). It seeks to avoid both idealism and materialism and the philosophical problems that attend them. William James advocated such a position (see radical empiricism), as did British philosopher Bertrand Russell (1872–1970). However, it has been difficult to convey satisfactorily what the nature of such a substance might be. Compare idealistic monism; natural monism. See also mind–body problem.