After several years of studying great thinkers such as Plato and Galileo, French philosopher, Rene Descartes, was left with more questions than answers. He found that philosophy only lead him to a position of skepticism as every question was being debated by philosophers. There were no absolute truths in philosophy.
Realizing that there was no way of being absolutely certain about any of his beliefs, Descartes was on a quest to find an undeniable truth.
Descartes believed that our senses are an unreliable way of finding the truth as our senses often deceive us.
When we are dreaming, it feels real. We wake up and realize that ours senses have deceived us and what we thought was real was in fact only a dream. Therefore, if my senses can deceive me in this way, I cannot trust my senses to give me the truth.
He then looked to science and mathematics. Can I prove absolute truths with mathematical questions? Then again, it is possible for God to deceive me into believing that these equations explain reality when in fact it has nothing to do with reality at all. Because of this possibility, I cannot be absolutely certain about the truths derived from mathematics or science.
After 20 years of meditating on this philosophical problem. Descartes found that he could doubt everything and almost gave up on his quest for absolute truth. Then he had an idea, an idea that would completely revolutionize the field of philosophy.
Descartes realized that: If I can doubt everything, even my own existence, there must be something doubting. I can only doubt my existence if I exist. Therefore I am absolutely certain that I exist.
"Cogito ergo sum."
I think, therefore, I am.