This classic is best known for kindling modern philosophy. Descartes questions any knowledge that is derived from our treacherous, human senses; in order to find truth, one must question what is "known." This is what has led many to realize that our ability to doubt is the result of our ability to think. It is the only certainty we have in life. Only our ability to reason can prove that we exist. Today, it's a principle that we can easily take for granted, but in its day, it sparked the deductive method that is so prevalent in our time.

René Descartes was a French philosopher, mathematician and physicist, considered the father of analytical geometry and modern philosophy.