  Reid's reductio of the Way of Ideas' theory of thought is intended to buttress Reid's claims that conceiving is irreducibly intentional and that, because of this, we have knowledge of the contents of our thoughts. Hume refrains from this claim in part because he sees no plausible metaphysics to support it. That is, if intentionality is endemic to thought, then the mind must not be what we think it is—a part of nature whose ideas operate on impact principles familiar from Newtonian mechanics. Reid is aware of this problem and prepares to bite the bullet. He describes our ability to apprehend one thing and not another as being “a natural kind of magic” (IHM 5.3, 59–60). In an effort to explicate this mysterious conceptual ability, Reid examines theories of thought of Aristotle and medieval philosophers. These theories posit substantial forms that conjoin thoughts to their intentional objects (Philosophical Orations, Reid 1937, 38 and Reid 1989, 66). Yet Reid judges them harshly and finds their ontology “incomprehensible” (EIP 2.8, 114). He withholds further speculation about the metaphysics that must undergird the claim that intentionality is primitive. This move illustrates a feature of Reid's philosophical method: common sense and knowledge are conceptually and explanatorily prior to metaphysical commitments.