René Descartes — Cogito, Ergo Sum

It is 1639, and a Frenchman sits at his desk to meditate. There is warmth from the flickering fire, casting his shadow on the wall. Shut off from the world, the meditator begins the systematic demolition of all his opinions through doubt.

He reflects on the numerous ways that the senses can mislead one: the sun appears small from Earth, and a straight stick looks bent in water.

Doubt may also be cast over the reliability of other sensory perceptions, such as that he is now sitting by the fire, dressed in a nightgown, feeling the warmth against his skin. After all, one sometimes dreams of playing in the sun while in reality one is asleep in bed; therefore, it is better to distrust the senses altogether.

The Frenchman is 44-year-old René Descartes. He wrote Meditations on First Philosophy (1641) while living in Holland—where he had settled at the age of 22.

Descartes then reasoned that even though mathematically two plus three makes five and a square has only four sides there was no ruling out the possibility of a creator who deceives one into believing that something is true when it is not—since one has already been deceived as with the bent stick.

Descartes then queried, that given everything was doubtful, could he state anything with certainty? In an inspired moment, he seized upon the fact that his mental faculties work (he is after all thinking these thoughts) as proof of the certainty that he exists.

Descartes stated his findings, the foundation of knowledge, as je pense, donc je suis, better known in Latin as cogito, ergo sum: I think, therefore I am.