Fallor, ergo sum
Nam et sumus, et nos esse novimus, et id esse ac nosse diligimus […] Nulla in his veris Academicorum argumenta formido, dicentium: Quid, si falleris ? Si enim fallor sum. Nam qui non est, utique nec falli potest, ac per hoc sum, si fallor. Quia ergo sum si fallor, quomodo esse me fallor, quando certum est me esse, si fallor? Quia igitur essem qui fallerer, etiamsi fallerer, procul dubio in eo quod me novi esse, non fallor. Consequens est autem ut etiam in eo quod me novi nosse, non fallar.
De civitate dei, book XI, chapter 26.
Yes, we exist and we know that we exist, and we love both that we exist and that we know it […] With respect to these truths, I am not at all impressed by the arguments of the Academicians (= skeptics of the Roman era) when they say: What if you are deceived (i.e. referring to sensorial il- and delusions)? For if I am deceived, I am (i.e. exist). For he who doesn’t exist, cannot be deceived; so through the same reasoning I exist, if I am deceived.. And since I exist if I am deceived, how am I deceived in believing that I am? for it is certain that I am if I am deceived. Since, therefore, I, the person deceived, should be, even if I were deceived, certainly I am not deceived in this knowledge that I am. And, consequently, neither am I deceived in knowing that I know.