Rene Descartes is known for his philosophical statement, "I think, therefore I am" originally in French, but best known by its Latin translation, "Cogito, ergo sum".
This is the philosophical principle that one's existence is demonstrated by the fact that one thinks and the intellectual process of self or ego.
Descartes began by observing that our human senses are deeply unreliable. Indeed if you doubt the reliability of senses, and even if you doubt the reliability of reason, these two being main sources of knowledge; Cogito remains true.