Rene DesCartes (1596-1650), lived a life focused on mathematics, philosophy and science during a period of history that was not particularly accommodating for analytical thought. DesCartes stated that he had no way of being certain that everything he perceived was nothing more than something he created subconsciously in his mind, such as a in dream. This theory fails because dreams are built on knowledge we have of the external world, on bits and pieces of our real experience. One of his many noteworthy accomplishments was that he claimed to have proven the existence of God. His theory states that "I have found within myself the concept of God".
DesCartes' concept of God was that he's an infinite, perfect, uncreated entity. Another premise states that the cause of his having the concept of God must be larger than the effect (the effect being his possession or perception of the concept). He asserted that cause must be larger than effect; therefore nothing must be larger than the concept of God, except for God. DesCartes said this proves the existence of God. He deliberated that God has it in his power to deceive us, even in matters of mathematical certainty. This theory fails because deceit is a matter of imperfection, therefore it would be impossible for a perfect entity to deceive us.
DesCartes and many of the people with whom he shared correspondence were the leading epistemologists of the time, and are still respected as some of the greatest philosophic minds of all time. DesCartes would begin an ontological argument by establishing an architecture metaphor that states that every individual's mind is built upon a system of axioms. This would mean that our conclusions and deductions are predicated on axioms, or things (situations, concepts etc.) we think we know to be truths. He would question things that most people would accept as truths by stating that it isn't really possible to be sure that what they perceive is the actual nature of reality.