In the Meditations, Descartes seeks to prove the existence of the external world in a series of five dialectically linked proofs. The first, in the so-called painter analogy of the First Meditation, investigates whether our ability to have dream representations (likened there to paintings of objects) presupposes acquaintance with material objects external to the mind, finding that it does not. What is presupposed, rather, is that there is a class of “simpler and more universal things … [which] include corporeal nature in general, and its extension; the shape of extended things.” These things “are as it were the true colour from which we form all the images of things” and play this role “whether they really exist in nature or not” (AT VII 20, CSM II 14).

The second and third attempts take place in the Third Meditation, comprising two different applications of causal principles to ideas, the first (paras. 8–11) to ideas considered “materially,” that is, as mental occurrences (“method 1”); the second (paras. 12–21) to ideas considered “representatively,” that is, as mental occurrences that represent one thing rather than another (“method 2”). The first method fails largely because of the possibility that there is inside me a capacity (“faculty”) for producing sensory ideas, as there apparently is in the case of dreams.

The second method contains the heart of Descartes’ causal epistemology, most perspicuously formulated in Axiom V of the Geometrical Exposition in the Second Replies: “The objective reality of our ideas needs a cause which contains this reality not merely objectively but formally or eminently.” The objective reality of an idea seems to be a reality that Descartes attributes to objects of our ideas regardless of whether those objects actually exist, and the formal containment of the reality is attributed to objects when they actually exist (see being, formal versus objective). There is a controversy about what “reality” refers to: Does it refer to a metaphysical category like being a finitesubstance or does it refer to a specific property, like being an intricate machine of a particular kind?