René Descartes was born March 31, 1596 in Touraine, France. He grew up to become one of the leading lights of Western Philosophy, and indeed has been dubbed by many the 'Father of Modern Philosophy'.
Descartes' key work was the Meditations in First Philosophy, written in 1641, which is still taught in many schools and universities today. Meditations introduces the method of "Cartesian doubt," which Descartes uses to attempt to find absolute knowledge.
Descartes on Absolute Knowledge
Descartes, a keen student of mathematics who would later also be hailed the founder of Analytical Geometry, wanted to put all of the sciences on the same firm footing of certainty and precision he saw in the foundations of mathematics. He wanted to find absolute knowledge, statements which absolutely could not be doubted. To aid this quest, he employed a system of methodological scepticism known today as "Cartesian doubt."
In order to find absolute truth, Descartes started out by dismissing all of the knowledge he had accrued in his life. He held this information as potentially false, especially the information that was collected through the senses. Descartes reasoned that as his senses had, on occasion, deceived him in the past, he had no guarantee that they would not do so again. Thus, he reasoned, it is impossible to ever be 100% certain that any knowledge gained through the senses is true. He needed to remove every possibly false presupposition from play before it would be possible to build up to absolute knowledge – in effect, he needed to start from scratch.
Descartes' Dreams and Reality
Descartes posited hypothetical examples to underline that the senses and past experience cannot be trusted. The first was the similarity between dreams and reality. It is extremely possible to have a dream which one believes to be reality, and in any case, the differences between the two are not quite sufficient for one to be 100% certain that one is not dreaming.
As Descartes puts it in the First Meditation, "How often does my evening slumber persuade me of such ordinary things as these: that I am here, clothed in my dressing gown, next to the fireplace - when in fact I am lying undressed in bed!"
In much the same way as the external senses cannot be fully trusted because they have a history of deception, Descartes believes that it is possible to have a dream which one believes to be reality one can never be one hundred percent sure that they are not dreaming.
Descartes' Philosophy of the Evil Genius
Descartes put forward a second hypothesis, one which goes even further than to dismiss the external world – it attacks the foundations of knowledge such as mathematics and science. Descartes invites the reader to suppose the existence of a God who is not entirely good, and is in fact an "evil genius, supremely powerful and clever, who has directed his entire effort at deceiving me". So every experience one has ever had could in fact have been the fabrication of this malign deity — much like the situation in the film 'The Matrix' — as unlikely as it seems, it is nonetheless a possibility, which rules out absolute certainty.
The Evil Genius could also be deceiving his victim about the laws and 'certainties' of science and mathematics – it is possible that 2+2 does not in fact equal 4, but that this is the invention of the deceiver. The same could be said of the rest of mathematics – the sum of the angles of a triangle, and so forth - and indeed the rest of the sciences. Given the possible existence of an evil genius, none of them represent absolute certainty.
Cogito Ergo Sum
By this point in the Meditations, Descartes found himself stripped of almost everything – the external senses, past recollections, even the mathematical proofs which inspired him to start his search for absolute knowledge in the first place – everything, it seemed, but his doubt. It was at this point that Descartes realised that he had stumbled upon his absolute truth; in order to doubt anything, he had to exist.
Likewise, in order to dream, or be deceived by the Evil Genius, he had to exist – in order to have any thoughts at all, he necessarily had to exist. Thus was born the famous phrase "cogito ergo sum," or "I think, therefore I am," and the foundation of absolute knowledge.
I Think, Therefore I Am
Later in the Meditations, Descartes attempts to build upon "cogito ergo sum" in order to re-establish certainty in other forms of knowledge, and even forwards an ontological argument for the existence of God, but it is this original statement which has endured as the focal point of the work. "I think, therefore I am", the phrase which breaks down Cartesian doubt, became one of the most famous philosophical expressions of all time, and established Descartes' position as the "Father of Modern Philosophy."
Descartes, R., Meditations on First Philosophy, Hackett Publishing, Indianapolis, 1993.