 Descartes’ ontological argument, first presented in the Fifth Meditation, aims to prove the existence of God from the idea of God (Descartes 1641, cited below from the edition by Adam and Tannery (1962–1976) and referred to as “AT”). Here is one way to formulate the argument (compare Pereboom 1996, 2010; for alternatives see the entry on  Descartes’ Ontological Argument):