Meditations on First Philosophy

Written in the first person, Rene Descartes's treatise asks readers to meditate along with him through six meditations, framed as if spread over six days, with each one calling the one before it yesterday. He begins by discarding every belief that falls short of absolute certainty, then attempts to determine what can be known with certainty. First published in Latin, its full title promises proofs that God exists and that the soul is immortal, and a dedication asks the Faculty of Theology in Paris to protect the work. The Meditations develop the metaphysical system Descartes first sketched in part four of the Discourse on Method. It remains among the most influential works of philosophy ever written.

Mark Manson listed it among the best philosophy books, describing it as the source of Descartes's famous I think, therefore I am.