René Descartes is known as the father of modern philosophy. His influences include Copernicus and Galileo. He founded his epistemology, the study of the theories of knowledge, on knowledge in god. He believed authority-based knowledge must be eliminated. He used reason in a rationalist approach to deduce fundamental truths. He began by using methodic doubt. Methodic doubt is based upon a model of analytic geometry and indicates reason alone is the only means of discovery. Doubting requires a necessary condition, which is a doubter. He often used proofs that resembled geometric proofs. Descartes held to know requires being certain and to reach certainty requires doubting everything.

First, he said sometimes the senses deceive, and occasions when they deceive cannot be distinguished from occasions when they do not. Thus, he concluded that any particular sense experience might be deceitful. Then, he stated that god is not the deceiver but some "malicious demon of the utmost power and cunning" uses all its energy and power to deceive. He was not saying he believed in an evil demon but considered the idea as a way for reason to discover the place to disprove it. Finally, he doubted dreams. He questioned how consciousness versus dreams was established. He also returned to the idea of doubting the senses because sensation also occurs in a dream state.

After employing methodic doubt, Descartes concluded that all thinking things exist. He coined "cogito, ergo sum" or "I think, therefore I am." He said that the evil demon could not hold both statements, "I exist" and "I do not exist," true at the same time. He said that he thought he existed but he was wrong about whether he did. He then stated that if the first is true, then he must exist to do the thinking, and if the second is true, then he cannot think he is thinking when he is not. Therefore, he concluded that because he was thinking that he existed, he existed. He thought, therefore he was. He also concluded that he could not doubt that he existed but that he could doubt whether things extended in space, such as bodies, existed. And thus, therefore he is not a body but rather a mind that could survive after the death of the body. Descartes arrived at the certain knowledge of "I think, therefore I am."

Descartes also developed a rational proof of the existence of god and god as not a deceiver. His entire epistemology was founded on this proof. Descartes began by saying that in his self, he has an idea of perfection, which implies a cause. (Causality operated along with the law of non-contradiction as a necessary assumption for self-consciousness.) He next indicated that the idea of his imperfection is derived from the ideas of perfection of god. His idea of perfection is a priori. He said his self-conception can be deduced from the idea of god but the idea of god cannot be conversely deduced. He believed the idea of god must be god because perfection necessitates perfection. Perfection comes only from perfection. Perfection cannot come from imperfection like x cannot be derived from a substance that does not contain x. (X is not in and cannot be derived from qwelkjnklsdfopujn.)

He continued that god provides a faculty of reason and does not deceive at every turn like the evil demon. The world, which comes from the idea of the world that comes from god, presents a true world and not a false world. God provides understanding to have knowledge. Knowledge is a priori. He finally concluded that god is trustworthy while the senses are not but now god provides trustworthy senses. The senses can now be trusted for knowledge.

Descartes then stated that error of judgement is placed on free will. Free will is the source of error. Error occurs from the misuse of judgement given by god. He thought reason to be infallible. Care must be taken and reason used to form ideas, which are clear and distinct. For example, the idea of god is clear and distinct through examination. God cannot be deceived just as humans are not deceived at every moment, only sometimes. However, imagination can be clear and distinct so just because something is known does not mean that something exists. Descartes believed perfection could not be without existence.

Descartes is considered a rationalist because his process of knowing moves from the mind to the world. He believes that by examining the mind, he could know what things are. Knowledge is a priori and arrived through deduction from innate ideas, fundamental laws, instinct. Knowledge exists before the senses. He relies upon reason and rational thought.

However, Descartes is accused of circular reasoning. He assumes in the premise the conclusion to be established. He states that god must exist for clear and distinct ideas. But, he must have clear and distinct ideas of god. He uses reason to establish reason.