What if reality is an illusion? How can you know if *you* aren't part of that illusion? If the world is a construct whose sole purpose is to fool your senses, what kind of conclusions can you draw about the nature of reality? Can you prove you exist? Can you even trust your thoughts? These very large questions if not dismissed consist hyprebolic doubt (aka Cartesian Doubt).
By asking and analyzing these very big questions, Descartes proved that you exist, and while not trustworthy, the mere fact you _have_ thoughts, proves it (Cogito Ergo Sum). Unfortunately, due to the high level of rigor and extreme doubt, it has proven impossible to build upon that very sound foundation, and his arguments trying to take it further do not express nearly the same level of rigor, and pale to his powerful first conclusions.
With the style of analysis and fearlessly examining this, he created the basis and foundation for most modern philosophy, since many schools of thought is based upon getting off his rigorous and rather lonely dead end island of "Cartesian Doubt" with a non-rigorous assumption or supposition.
The book is a fast, and intense read, appearing to have been written over a few days. The reader is taken along for the ride and in my case, my mind was blown at the level of rigor. To me his argument leading to "Cogito Ergo Sum" is as close to a bulletproof, rigorous, perfect argument that you can experience. Its only weakness, though, if you stick to that level of rigor, you really cannot prove anything else besides your own existence!
Definitely worth the price of admission. Especially to non-philosophers like myself!