This is an important work to read, even though it is completely wrongheaded. The argument will try to suck you in, building a metaphysic from the most basic building block Descartes could imagine: the reality of his own thoughts. “Cogito ergo sum” is the famous line we all know: I think therefore I am.
The problems Descartes creates for himself are legion, but the biggest is the circularity of his argument. The book is good, it states the dualist position well, but Descartes was a better mathmetician than a philosopher.