  As usual, Reid builds his theory by learning from mistakes of earlier theories. Medieval philosophers argued, by Reid's account, that a universal exists independently of individual things. Some of these philosophers argued that the independent existence of a universal was existence as a form, a venerable philosophical term. Addressing this, Reid says it is impossible that “a triangle should really exist which has no precise proportion of sides and angles” and, generally, it is “impossible that any being should exist which is not an individual being” (EIP 5.6, 396). The principal philosophical error that led to the reification of universal properties as independent existents is the conflation of the objects of conception with the objects of perception (EIP 4.1, 302–3). The metaphysics required to undergird the existence of universals is anathema to Reid's commonsense commitments and his empirical method.