Born in Thagaste, North Africa, on November 13, 354, Augustine studied rhetoric at the University of Carthage and taught rhetoric, both in North Africa and later in Italy. As a young adult he was a Manichaean “hearer,” but he became disillusioned with Manichaeanism. At twenty-nine he sailed for Italy, where he lived for about five years. In Milan he was given a Christian baptism by Bishop Ambrose. After the death of his mother, Monica, he returned to North Africa, where he spent the rest of his life and became bishop of Hippo Regius. His treatises, biblical commentaries, sermons, and letters constitute the largest body of writing we have from any ancient author.

In the Fourth Objections, Arnauld identifies Augustine's De libero arbitrio 2.3 as a source for Descartes’ cogito (AT VII 197–98, CSM II 139). Later in his Objections Arnauld offers support from two of Augustine's works for Descartes’ claims about the certainty of reason. Still later, Arnauld recommends Augustinian support for the claim of God's “eternally present existence” (AT VII 211, CSM II 148). Finally, Arnauld offers a warning from Augustine that “absolutely nothing in human society will be safe if we decide to believe only what we can regard as having been clearly perceived” (AT VII 217, CSM II 152). Descartes, in his reply to Arnauld, thanks him for “bringing in the authority of St Augustine to support me” (AT VII 219, CSM II 154) but acknowledges no debt to Augustine or any need to respond to his views. However, Descartes does thank Colvius, in a letter, for calling his attention to a passage in Augustine relevant to the cogito. He adds that he went to the town library to check it out, as if it were new to him (AT III 247, CSMK 159). However, recent scholarship (see especially Menn 1998) reveals the extensive debt Descartes owes to Augustine. Certainly something very close to Descartes’ cogito is to be found in Augustine's City of God 11.26 and in his De trinitate 15.12.21, as well as in the De libero arbitrio passage Arnauld mentions. But there are other important similarities as well.