This paper reconstructs (in natural language) and discusses a proof of God’s existence by Anselm of Canterbury’s friend Ralph of Battle (1040–1124), developed in his recently edited De nesciente , a fictitious dialogue between a Christian and an atheist. Without precedent in antiquity and the Middle Ages, Ralph’s proof has never been examined in detail. It combines a “cogito” argument with a two-part cosmological argument. The paper first presents the textual basis and an exegetical interpretation of Ralph’s reasoning, classifies the parts of the proof historically and systematically, and then compares these with the proofs of God’s existence as well as other arguments in Anselm’s Proslogion and Monologion . Finally, it points out some similarities between Ralph’s “cogito” argument and a passage in the Liber pro insipiente , which may suggest that this anonymous critique of Anselm’s Proslogion proof was authored not by Gaunilo, as traditionally thought, but by Ralph.