I've spent a lot of time rejecting Descartes, because I've found such deep critiques of rationalism as to convince me it's one of the greatest intellectual sins of the modern world. You cannot build up all knowledge from nothing; starting from absolute doubt, you will get nowhere, and wherever you think you've gotten, you're secretly drawing from the assumptions you were supposed to have rejected. Human knowledge is heavily dependent on tradition, imitation, and intuition, and Cartesian rationalism seems to me a form of intellectual self-flattery with no solid foundation.
Then again, we often rebel the hardest against that which is closest to our hearts. In the classic statement, "I think, therefore I am," I've always seen a most profound and courageous kind of insight. Even if nothing else is certain, there is still one thing of which I'm absolutely sure: I exist. And the reason I am certain of that is the very act of thinking. Do I exist? If I have the ability to ask the question, then surely the answer is yes.
If there is any weakness in this argument, it's in the subject, not the verb. Normally when I use the word "I," either in public communication or private thought, it is to distinguish an inner and an outer world. I have thoughts which, I presume, are hidden from others. The world "out there" might be totally mysterious and deceptive, but at least I am sure of my own inner life.
But why should I think that? What gives me the right, a priori, to distinguish between "inner" and "outer"? That is a metaphor imposed on me by the language I use; it is not an inevitable deduction made from experience. What is certain is that thought occurs. If others claim not to be able to see these thoughts--"my" thoughts--happening, that will perhaps lead me to claim the thoughts as "mine." But if I take nothing for granted, if I seek to go back to first principles, then the only thing absolutely sure is that something is happening. Thought occurs. Therefore something--everything--must exist. That is to say, there exists an "everything" which really is there. And I arrive at that conclusion by the mere act of thought.
That is to say, every act of thought finally reposes on the firm foundation of God's very Name--"I Am." This is not an individual I, not an interiority or a selfish ego. It is, rather, existence itself.
The most profound mystery of life is neither its origin nor its ultimate destiny nor even its purpose. It is the fact that it exists. I've always wondered why "the fear of the Lord is the beginning of wisdom and knowledge." That "fear," it seems to me, is the overwhelming awe one experiences at the very thought that something exists at all. If you start from there, everything else is trivial by comparison.