Descartes’ Third Meditation presents an alleged proof for the existence of God that proceeds from the existence of an idea of an infinite being, God—an idea with infinite objective reality—to the existence of God himself. There is a tendency to understand the meditator as simply assuming the premise that he has an idea with infinite objective reality, or alternatively, as drawing it from the reach of introspection and the transparency of thought. Either way, readers of the Meditations often find the premise unmotivated, and do not take Descartes to provide any argument for it. This paper aims to show that Descartes does provide an argument for the premise. My interpretation focuses on the meditator’s evolving conception of his idea of an infinite being in the Third Meditation. In so doing, the interpretation highlights a way in which epistemic progress is achieved in the Meditations, namely, through a process of correcting misconceptions.