Descartes’ cosmological (or causal) arguments for the existence of God are often thought to be among the least plausible aspects of his philosophy and, perhaps as a result, have been relatively neglected by commentators. But these arguments are as important to his epistemic project in the Meditations as they are controversial. Having proven in the Second Meditation that he exists as a thinking thing, the meditator aims in the Third Meditation to discover whether anything outside of him exists. Before philosophizing, he had judged that there are physical objects that cause his sensations, and that these sensations resemble their causes, but he now recognizes that such judgments are based on “blind impulse” and thus untrustworthy (AT VII 38–40, CSM II 26–27). Descartes will eventually develop a proof of the existence of bodies (in the Sixth Meditation), but he postpones that here, turning his sights on the more ambitious project of demonstrating God's existence (see body, proof of the existence of). His goal is not just to overcome solipsism but to chart a course that will enable him to dispel the hyperbolic doubts of the First Meditation – most notably, the “omnipotent deceiver hypothesis” – by proving that he was created by an all-perfect being who would not deceive him.

Descartes presents two theistic proofs in the Third Meditation, though most commentators treat the first as the main argument, and in a letter Descartes asserts that the two arguments are “reducible to one” (AT IV 112, CSMK 232). Given these considerations and the constraints of space, the main focus here will be on the first argument. The main problem with this argument is that it relies on seemingly bizarre principles of causality and antiquated doctrines about degrees of reality and different kinds of being, which have their source in Neoplatonism and Scholasticism and do not seem to be a good fit within Descartes’ own metaphysical system. How can such principles enable us to achieve knowledge if they themselves are so dubious? In the Objections and Replies, Descartes responds to several criticisms of his arguments. He also reformulates his cosmological arguments in the Geometrical Exposition and in the Principles (1644), though he presents them there after the ontological argument, reversing the order he had followed in the Meditations, leaving commentators to debate the significance of this reordering.