When Descartes’s friend Marin Marsenne read the Discourse on Method he pointed out the similarity between Descartes’s cogito argument and an argument of St. Augustine’s in the City of God, XI, 26. Against the suggestion that he might be mistaken about his own existence, Augustine writes:

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.

It has now become standard to note the similarity between Descartes’s argument and Augustine’s.

But no one seems ever to note the similarity between the cogito argument and certain arguments proposed by St. Thomas. Consider De veritate, q. 10, a. 12, for example. In this article Thomas asks whether God’s existence is per se notum. In the seventh objection we read this:

…verius esse habet Deus quam anima humana. Sed anima non potest se cogitare non esse. Ergo multo minus potest cogitare Deum non esse.

Thomas replies in the following way:

[C]ogitari aliquid non esse, potest intelligi dupliciter. Uno modo ut haec duo simul in apprehensione cadant; et sic nihil prohibet quod aliquis cogitet se non esse, sicut cogitat se quandoque non fuisse. Sic autem non potest simul in apprehensione cadere aliquid esse totum et minus parte, quia unum eorum excludit alterum. Alio modo ita quod huic apprehensioni assensus adhibeatur; et sic nullus potest cogitare se non esse cum assensu: in hoc enim quod cogitat aliquid, percipit se esse.

So, no one can assent to the thought that he does not exist because in the very act of thinking he perceives that he exists. (The same pattern of argument can be found in De veritate, q. 10, a. 8, ad 5 and Summa contra gentiles, III, c. 46.) Unlike Descartes, however, Thomas does not make this truth into a first principle. For Thomas, being (ens) is the first principle because, he says, it is the first thing that is “most evident” (notissimum) to us (De veritate, q. 1, a. 1).

(I originally posted this on the AMU Philosophy Department blog last month. I believe in recycling. It’s good for our planet.)