Book Review: The Selections from the Principles of Philosophy

This book, by René Descartes, is airing the author's thoughts on a variety of issues of philosophy. The first part of the book begins with the principles of knowledge, the significance of doubt and the role of free will, the absolute need of a god and the necessity for examining the secrets of creation, and how our mistakes are our own while our good deeds are not. The second part deals with the principles of things, where it pontificates on issues like the grounds on which the existence of material things may be known with certainty. How space is not in reality different from corporeal substance, how the world is indefinite, and what motion is, and many other questions of such nature are discussed here. The third part discusses the world that we view. Why we need not think too highly of the works of God, why all things can be thought to be created for the sake of man, are a few questions covered here. Next and final part looks at the earth, how should we perceive, what we perceive. The book concludes with a statement, 'I desire no one to believe anything I may have said, unless he is constrained to admit it by the force and evidence of reason'.
I have come across this quote, I think, therefore I am, many a time. Almost always for justifying one's limitless foray into thoughts, while rest of the mankind waits for some action. I found in this book, rather than thinking, the author gives great importance to the result of thinking, doubts. Doubts are specific to humans, the book proclaims. 
It is as though 'I doubt, therefore I am' should have been the idea we grasped. Had we done so, we would have been seeing a different world now.