Augustine of Hippo1979

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, quo modo esse me fallor, quando certum est me esse, si fallor.

What difference, if you are mistaken? For if I am mistaken, I am. For he who is not, assuredly cannot be mistaken; and therefore I am, if I am mistaken. Therefore because I am if I am mistaken, how am I mistaken that I am, when it is sure that I am, if I am mistaken.'

The Latin variations of the statement sum si fallor (I am because I err), have sometimes become paraphrased Fallor, ergo sum (I err, therefore I am), based on the form of the later Cogito, ergo sum (I think, therefore I am) of Principles of Philosophy (1644) by Descartes. Familiarity with Augustine's thought could have actually inspired some of Descartes statements here and in the earlier Meditations on First Philosophy (1641):
"Doubt is the origin of wisdom.".