  The first rule does not entail that unobservables like ether, subtile fluids, or forces do not or cannot have explanatory force, let alone that unobservables do not exist. Reid is explicit that questions about the existence of ether were open questions that were not settled by a priori (EIP 2.3, 82). Physical forces and causes do exist and may yield their secrets to science in the form of purely physical forces of attraction (An Inquiry into the Human Mind on the Principles of Common Sense, abbreviated to ‘IHM’, 7, 211). This has two corollaries of considerable importance for understanding Reid's methods. First, the fact that Reid admits that these forces of attraction—unobservable, immaterial forces—may be physical implies that he is comfortable with a physical explanation of gravitation that does not entail the existence of God. Second, physical forces will not yield their secrets to an impoverished Cartesian science. This criticism is made on the grounds that Cartesians were attempting to understand gravitational attraction in terms of only extension, figure and motion—for Reid, a catastrophic mistake. In fact, Reid's criticism of the Cartesians has provided a clue for an account of Reidian forces as irreducible properties of matter (Callergård 2005, 2010).