Can anything be known to be certain? This is a more difficult question than most people might recognize. Rene Descartes says yes and presents us with one of the most elegant thought experiments in the history of philosophy. We begin by calling into doubt all claims of "knowledge"; believing nothing that cannot be affirmed with absolute certainty:
Imagine now that an all-powerful, all-knowing being might exist external to that which we can experience with our senses, i.e., external to the material world (recall that we can neither know this nor know otherwise). Imagine further that this extra-material entity may be a devious trickster, messing with my mind, perhaps to amuse a twisted sense of humor. Because the possible trickster would exist external to the access of scientific scrutiny, I could, in my state of absolute skepticism, never know whether this sadistic consciousness is at work, not only in the material world, not only in my conscious perception of the material world, but in fact in the perceptions of all other conscious beings as well. Thus all scientific proofs might be mere illusion and there could be no means of determining this. In other words, if all material objects and all subjects of thought are inherently uncertain, and this is indeed a logical conclusion at this point in our consideration, what then could be known with certainty? Is then the only absolute certainty this universal and impenetrable uncertainty? Could it ever be truly known that anything exists apart from the possibility of the trickster? Only one thing: that [without regard to whether or not it is being deceived] the mind of the thinker must exist, for otherwise there is not even the illusion that our consideration is happening. Thus the only thing that I may know beyond any doubt is that my mind does exist. Cogito ergo sum, i.e., "I think, therefore I am." This singular certainty is not without further implications. For while we have established that consciousness (i.e., mind) is more certain to exist than is matter, we don't know why this should be true. Or do we? Descartes says that there is a reason we must reach this conclusion and presents his ontological argument for the existence of a perfect and beneficent Mind beyond material constraints and uncertainty (that mind being God).
Whether or not Descartes believed he had "proved" the existence of God is not a very interesting point (apparently he thought so). As Pascal pointed out, such proof -- or disproof -- is not possible within the inherent limits of human investigations (Pascal found nature and reason to powerfully infer God's existence in a probabilistic sense, while "scientific" proofs must be uncertain, uncertainty being the nature of corporeal existence). What Descartes did "prove" is that the idea of an extra-cosmic mind is a rational conclusion (and is rational to a greater extent than any phenomenological observation that we might assume to be "true"). Some claim to rebut Descartes' ontology citing his geometric analogy, which was based in the Cartesian paradigm of his day. This is no great difficulty however, another mathematical illustration might have been developed had Descartes knowledge of 21st century mathematics. In fact, Descartes asserts that his conclusion does not rest on his understanding of geometry (which was about to be overtaken by Newton's mathematics). He believes that he could provide "an infinity" of allegories to illustrate his ontology. Here we find an expression of how Descartes' struggle with vanity leads to some hasty proffers (finite beings cannot wholly examine an infinity, even if we accept the existence of such). Many other thinkers, who agreed with some of Descartes arguments, quickly took umbrage with his more disputable statements. Descartes then rebutted these rebuttals. In fact some of these arguments continue today. Such is Descartes' importance to [some say "modern"] philosophy.
There are still other interesting aspects to these essays: Descartes' method (which is sound), his interest in medicine, physiology, neurology, his anticipation and analysis of "artificial intelligence" (three centuries before science fiction writers 'invented' the idea). Also interesting is the author's plea to the public (the work is clearly addressed to a general readership and not to his nemesis, the Jesuits, as some reviewers mistakenly suggest). Noticeably struggling to maintain his humility, the brilliant Descartes asks to be left to his work in physics. Rather than taking precious time to explain and defend his theories, he wishes to be left alone to focus on his work, asking to be judged and explained by it after his death.
"A Discourse on Method: Meditations and Principles" is more than a book, it is a challenging and rewarding mental experience. It is a tough read but well worth it just to read "I think, therefore I am" in its proper context (the simple statement that Descartes considers his first principle of philosophy).
The book is divided into three parts. In "A Discourse on Method," Descartes lays out his first principle of philosophy, and his plan for rejecting false assertions and deriving true principles. The "Meditations on the First Principle" is the wide ranging essay where "I think, therefore I am" is expanded to include all of its implications. These implications are wide ranging, from the existence of God, to the existence of our bodies, other physical objects, various scientific principles, and finally, whatever we are able to know as truth. Here is where the book poses its greatest challenge. At this point I was only reading 2-4 pages at a time. Then when I finished this part, I went back and reread a bulk of it to fully grasp the key points of the "Meditations." The third part, "The Principles of Philosophy," wouldn't have been so difficult if my brain hadn't been taxed as it was by the "Meditations." But the Principles are well organized and clearer, making the book more satisfying to read again.
Overall, this book is a treasure as an intense mental revelation. It brings together Descartes' best writing for the general reader, if the reader is up to the challenge.