The one thing people are sure to know about Descartes—who know anything about him at all—is that he said (approximately), “I think, therefore I am.”
Therefore, it is ironic that Descartes was not the first to say this. Consider the following:
…I am most certain that I am and that I know and delight in this. In respect of these truths, I am not at all afraid of the arguments of the Academicians [i.e., skeptic philosophers], who say, “What if you are deceived?” For if I am deceived, I am. For he who is not, cannot be deceived; and if I am deceived, by this same token, I am. And since I am 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.
This passage is from St. Augustine’s City of God (XI.26). The reference to the Academics is significant. Augustine was evidently aware of philosophical skepticism and concerned to answer it at least in some respects—just as was Descartes. Moreover, Augustine contrasts the frailty of sense-perception as a means of knowledge with the certainty of “another and far superior sense, belonging to the inner man” (XI.27)—just as does Descartes. Immediately preceding the passage quoted above, Augustine asserts that our knowledge of our own existence is not subject to illusion just because it is not derived from the bodily senses, which only produce in us images resembling sensible objects, not the objects themselves. Intellectual knowledge, such as the knowledge we may demonstrate of our own existence, is free of “any delusive representation of images or phantasms.”
Clearly, Augustine’s cogito was not merely a transient thought which he failed to fully recognize or develop. According to Roy Sorensen (A Brief History of the Paradox, Oxford U.P., 2003, p. 170), Augustine presents the argument no less than seven times in assorted works. (Some other references I’ve found are The Trinity (10.10.14) and the Enchiridion (7.20).) Of course, Augustine does not employ his cogito as the foundational argument for an elaborate epistemology and system of knowledge. Still, in view of the fame the cogito argument acquired with Descartes, it seems odd that Augustine’s prior claim is so little known.
Did Descartes rip off Augustine? He claims not. However, Sorensen is not convinced, pointing out the ubiquity of the cogito in Augustine’s writings and the popularity of Augustine with Descartes’s Jesuit teachers at La Flèche.
In any event, Descartes employed his cogito in one important way that Augustine did not. Specifically, he thought it demonstrated the prior certainty of consciousness, by which I mean loosely that the mind is better known than the body. For example, a visual experience as of an object—a cup, say—in front of me may or may not really be of a cup, but it is unquestionably an experience. Descartes realized that only “thought,” by which he meant to include any conscious event, could support his argument. It wouldn’t do, for instance, to say, “I walk, therefore I am,” even though walking supports the logic of the argument, as presented by Augustine above, just as well as being deceived. Try substituting “walk” for “be deceived” in the passage above. Except where being deceived refers specifically to being deceived about one’s own existence, as opposed to being deceived in general, the logic of Augustine’s argument remains undisturbed. Thus, the fact that the event in question is being deceived appears to be almost incidental in Augustine’s cogito. But it isn’t; only an event involving conscious “thought” will support the argument. And this implies that the reality of thought is more securely knowable than the reality of the objects of thought. This is a point that Augustine may have missed, even though, with his talk of images and phantasms, he had the conceptual tools that would have enabled him to do so.
The prior certainty of consciousness, after going more or less unchallenged for nearly 300 years after Descartes, finally came to be controversial and to be rejected by many philosophers. But I am inclined to think it is true and that the cogito provides a pretty good argument for it. (Of course, the further uses Descartes made of the prior certainty of consciousness, for instance as support for mind–body dualism, are another matter entirely.) In this respect, Descartes saw more deeply into the cogito, even if he did not invent it.