René Descartes 
Meditations on First Philosophy [EPUB ebook] 
A Philosophical Treatise in Which the Existence of God and the Immortality of the Soul Are Demonstrated

’Meditations on First Philosophy’ is made up of six meditations, in which Descartes first discards all belief in things that are not absolutely certain, and then tries to establish what can be known for sure. He wrote the meditations as if he had meditated for six days: each meditation refers to the last one as 'yesterday’. 'Meditations on First Philosophy’ is one of the most influential philosophical texts ever written and it is widely read to this day.