Only a philosopher would spend time trying to answer the question, "do I exist?"
It's not that philosophers are worried about the answer. They know that they exist. But given that the answer is automatically "yes", how do we prove that answer? It's not enough to answer a question - you must offer evidence to support that answer. What evidence can you offer in order to convince me that you exist?
Descartes and Augustine share not only the argument Cogito ergo sum - in Augustine Si fallor, sum - but also the corollary argument claiming to prove that the mind (Augustine) or, as Descartes puts it, this I, is not any kind of body. "I could suppose I had no body," wrote Descartes, "but not that I was not", and inferred that "this I" is not a body. Augustine says "The mind knows itself to think", and "it knows its own substance": hence "it is certain of being that alone, which alone it is certain of being." Augustine is not here explicitly offering an argument in the first person, as Descartes is. The first-person character of Descartes's argument means that each person must administer it to himself in the first person; and the assent to Augustine's various propositions will equally be made, if at all, by appropriating them in the first person. In these writers there is the assumption that when one says "I" or "the mind", one is naming something such that the knowledge of its existence, which is a knowledge of itself as thinking in all the various modes, determines what it is that is known to exist.
But Descartes recognized that his use of this form of argument is quite different from Augustine's: "I do indeed find that Augustine does use it to prove the certainty of our existence. He goes on to show that there is a certain likeness of the Trinity in us, in that we exist, we know that we exist, and we love the existence and the knowledge we have. I, on the other hand, use the argument to show that this I that is thinking is an immaterial substance with no bodily element. These are two very different things."
Augustine's purpose in the larger context is to establish the continuing goodness of the world following the fall. To this end, Augustine argues that "we recognize in ourselves ... an image of God, that is of the Supreme trinity. It is not an adequate image, but a very distant parallel." And this premise leads to the conclusion that "we are human beings, created in our Creator's image." Thus, for Augustine, "Self-certainty thus leads self-consciousness back to the inner consciousness of God, which is found to be more essential to consciousness than itself. For the si fallor, sum does not aim at the ego, nor does it come to a half in the res cogitans, seeing as the interior intimo meo transports it, as a derived image, toward the original exemplar. The si fallor, sum remains the simply, though first, moment of a path that, in two other more rich moments (knowing one's Being and loving it), disappropriates the mind from itself by the movement of reappropriating it to its original, God. The si fallor, sum does not assure the mind of having its principle in itself, since it does not grant it Being in itself nor saying itself by itself (like substance). On the contrary, si fallor sum forbids the mind to remain in itself, exiled from its truth, in order to send it back to the infinite original. The mind is retrieved only insofar as it is exceeded.
Descartes wants to show that "by means of the certainty of Being that thought secures for what from now on becomes an ego" that the I is an immaterial substance: "What is at stake, then, is not found simply in the connection of thought and existence, however certain this connection might be. That the mind thinks, therefore that it is insofar as it thinks – this belongs to an inference that is if not banal ... at least quite commonplace. What is peculiar to Descartes consists, as he so lucidly indicates, in interpreting the certain and necessary connection of the cogitatio and existence as establishing a substance, and moreover a substance that plays the role of first principle."
We may conclude that, despite the rather different goals of their writings, Augustine anticipated Descartes by over a thousand years, and even anticipated Anselm by five hundred years, in composing what amounts to an a priori argument directed against radical skepticism. For, although Augustine's argument makes reference to sensation, the structure of his argument is essentially a priori.