Descartes is most well known for his “I think, therefore I am.” Bent on finding certainty upon which to build his beliefs and non-basic knowledge, Descartes wielded his method of doubt like a machete, undercutting all of his beliefs, allowing them to fall into uncertainty. Like the skeptics, Descartes was perhaps reacting against the various competing truth claims of his day, and resorts to doubt as his answer to the problem. Unlike the skeptics, though, Descartes doesn’t use the method of doubt to undercut certainty and knowledge; rather, doubt serves as his route to certainty. Hence, his “I think, therefore I am” grounds certainty in the very fact that he is doubting, and because he knows that he doubts, he can know certainly that he exists. From this vein of though, Descartes is able to develop his version of the Ontological Argument for God’s existence and to distinguish between foundational and non-foundational beliefs (I realize that this discussion is broad in nature and misses out on the nuances of Descartes’ method; my purpose, though, is to provide a basic picture of his method. To do otherwise would bog down this post).
Roughly 1100 years prior to Descartes, Augustine arrives at the certainty of his existence in a similar manner as that of Descartes. In City of God, Book XI.26, Augustine discusses how the image of the Trinity is recognized in humanity to an extent. The image of the Trinity in mankind is not “adequate,” but is a “very distant parallel” (City of God XI.26, trans. Bettenson). He further states: “We resemble the divine Trinity in that we exist; we know that we exist, and we are glad of this existence and this knowledge” (XI.26; emphasis mine). That one can have “the certainty that I exist…is independent of any imaginary and deceptive fantasies.”
This certainty of one’s existence is contra the philosophy of the Academics who question whether one can be mistaken or deceived about his own existence. Bettenson, in footnote 48 (p. 460) notes that Augustine here refers to the “philosophers of the ‘Second Academy’ who followed Arcesilaus of Pitane…in adopting the scepticism (sic) of Pyrrhon of Ellis.” In response to the skepticism of the Academics regarding one’s existence, Augustine states:
In respect of these truths, I am not at all afraid of the arguments of the Academicians, who say, What if you are deceived? For if I am deceived, I [exist]. For he who [does not exist], cannot be deceived; and if I [exist] deceived, by this same token I [exist]. And since I [exist] if I am deceived, how am I deceived in believing that I [exist]? For it is certain that I [exist] if I am deceived. Since, therefore, I, the person deceived, should [exist], even if I were deceived, certainly I am not deceived in this knowledge that I [exist]. And, consequently, neither am I deceived in knowing that I know. For, as I know that I [exist], so I know this also, that I know. And when I love these two
things, I add to them a certain third thing, namely, my love, which is of equal moment. For neither am I deceived in this, that I love, since in those things which I love I am not deceived; though even
if these were false, it would still be true that I loved false things. For how could I justly be blamed and prohibited from loving false things, if it were false that I loved them? But, since they are true
and real, who doubts that when they are loved, the love of them is itself true and real? Further, as there is no one who does not wish to be happy, so there is no one who does not wish to be. For
how can he be happy, if he is nothing? (trans. Marcus Dods, ec. Philip Schaff, http://www.ccel.org/ccel/schaff/npnf102.html)