St. Augustine from The City of God on the nature of doubt:

I am quite certain that I am, that I know that I am, and that I love this being and this knowing.
Where these truths are concerned, I need not quail before the Skeptics when they say.... "What if you should be mistaken?"
Well, if I'm mistaken, I exist.
For a man who does not exist can surely not be mistaken either,
and if I am mistaken, therefore I exist.