Rene Descartes 
Meditations on First Philosophy [EPUB ebook] 

"Meditations on First Philosophy, " authored by René Descartes and published in 1641, is a foundational work in modern Western philosophy. In this series of meditations, Descartes engages in a process of radical doubt, systematically questioning the validity of his beliefs to establish a secure foundation for knowledge. He ultimately arrives at the famous conclusion, "Cogito, ergo sum" ("I think, therefore I am"), affirming the existence of the thinking self as the first indubitable truth.