Here's a nice quote from this blog:
Descartes said cogito ergo sum (‘I think therefore I am”) but Augustine anticipated the thought by a millenia or so with his own version of the cogito. Augustine’s version really says amo ergo sum–we love therefore we are.
We resemble the divine Trinity in that we exist; we know that we exist, and we are glad of this existence and this knowledge. In those three things there is no plausible deception to trouble us. For we do not apprehend those truths by the bodily senses by which we are in contact with the world outside us – perceiving colour by sight, sound by hearing, odour by the sense of smell, flavours by the taste, hardness and softness by touch. We can also summon up in thought the immaterial images which closely resemble those material things apprehended by sense; we retain them in our memory; and through those images we are aroused to desire the things they represent. But the certainty that I exist, that I know it, and that I am glad of it, is independent of any imaginary and deceptive fantasies.
In respect of those truths I have no fear of the arguments of the Academics. They say, ‘Suppose you are mistaken?’ I reply, ‘If I am mistaken, I exist.’ A non-existent being cannot be mistaken; therefore I must exist, if I am mistaken. Then since my being mistaken proves that I exist, how can I be mistaken in thinking that I exist, seeing that my mistake establishes my existence? Since therefore I must exist in order to be mistaken, then even if I am mistaken, there can be no doubt that I am not mistaken in my knowledge that I exist. It follows that I am not mistaken in knowing that I know. For just as I know that I exist, I also know that I know.
And when I am glad of those two facts, I can add the fact of that gladness to the things I know, as a fact of equal worth. For I am not mistaken about the fact of my gladness, since I am not mistaken about the things which I love. Even if they were illusory, it would still be a fact that I love the illusions. For how could I be rightly blamed and forbidden to love illusions, if it were an illusion that I loved them? But since in fact their truth is established, who can doubt that, when they are loved, that love is an established truth? Moreover, it is as certain that no one would wish himself not to exist as it is that no one would wish himself not to be happy. For existence is a necessary condition for happiness.Augustine, City of God, XI.26