Meditations on First Philosophy by René Descartes is a book where he questions everything he thinks he knows to find absolute truth. He starts by doubting everything—even his senses and reality itself—until he reaches one thing he can't doubt: the fact that he is thinking. This leads to his famous idea, "I think, therefore I am." From there, he tries to prove that God exists and that the physical world is real. He argues that because God is perfect, He wouldn't trick us into believing in a false reality. By the end, Descartes rebuilds his understanding of the world based on reason, logic, and the belief that clear and distinct ideas must be true.