   Another area of controversy in Reid's empiricism involves the grounding of his belief to the effect that ether does not exist. Ether appears to represent just the sort of posit (originally by Descartes in Principles of Philosophy) that Newtonians often enjoyed sweeping into the trash. Lacking any observational evidence for ether, Descartes posits ether as a medium through which forces can act on bodies that are not in direct contact. But does Reid reject ether theory on the grounds that it is unobservable and therefore does not belong in a Newtonian science, or does he reject ether theory because scientists as yet lack justificatory observations on its behalf? Reid's apparent hostility to “hypotheses” in philosophy and “efficient causes” in natural philosophy gives rise to the belief that Reid does not believe that forces should be allowed into philosophy. The answer to this question can resolve that issue and assist in locating Reid's philosophy of science on the positivist/realist spectrum.