Discourse on the Method of Reasoning
Most philosophers don't get in an immortal tagline like Descartes' "I think therefore I am," but don't go thinking he was all about succinct one-liners, as the title of his most famous work reveals. It's referred to as the Discourse on the Method, but it's actually: Discourse on the Method of Rightly Conducting One's Reason and of Seeking Truth in the Sciences.
Discourse on the Method contains 6 parts: Part 1 is the introduction, where Descartes casts off all other learning to reexamine the nature of knowledge. Part 2 he lays out the rules for his life of introspection. Part 3 he lays out the plan for a virtuous life. Part 4 is the heart of the work: Reject anything you've ever been taught and construct your own system of reason and virtue. And parts 5 & 6 are pretty weak. So abandon all that you know as true and get down with the Discourse.
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