In developing his first and, in many ways, most central argument for God's existence in the Third Meditation, Descartes invokes a distinction between two ways in which things may be said to be: formally and objectively. Formal being consists in the reality something possesses in virtue of existing; objective being consists in the reality something possesses whenever there is an idea of it. God is said to exist objectively in our idea of him, and while objective being or “being in the intellect” is generally diminished compared to the formal being of something existing outside the mind, it is not thereby nothing. Since it is “not nothing,” the objective reality of an idea stands in need of a cause (AT VII 41–42, CSM II 28–29). The infinite degree of objective perfection of the idea of God implies God's existence (AT VII 45–52, CSM II 31–36). That this argument hangs on a rather dubious distinction between two modes of being was not lost on Descartes’ contemporaries. Johannes Caterus responded that the Schoolmen would not take “objective being” to imply a mode of being distinct from formal being but simply the “extrinsic denomination” of a thing by the intellect (AT VII 92–93, CSM II 66–67). This is our first inkling that Descartes is not inventing the terminology and that there is no general consensus about how it was to be understood. Caterus represents one interpretation, but there were others in what was in fact a protracted medieval debate. Descartes’ use of the terminology is obscure – “objective reality,” “objective being,” “objective intricacy,” and “objective perfection” may seem to connote differently but are used interchangeably (see AT VII 161, CSM II 113–14; AT VIIIA 11, CSM I 198; AT VIIIB 362–63, CSM I 306) – and his indebtedness to tradition difficult to discern. We can only do so much in reconstructing the peculiar way in which he intends the distinction to be understood.

The Third Meditation purports to be “of God” but contains both a theory of ideas and an account of how ideas may be true or false. The two turn out to be connected because it is of the nature of ideas to be “as if images of things” (AT VII 3, CSM II 25–26; AT VII 43–44, CSM II 29–30), and in purporting to represent something, an idea may thus be true or false (see falsity, material).