Knowledge for the rationalist is what can be deduced from principles that cannot be otherwise. These facts are considered to be undoubtable or indubitable. Examples of these kinds of principles include statements such as: Bachelors are unmarried males. A thing cannot be and not be at the same time in the same way. Triangles have three sides. A whole is always greater than any one of its parts.
These kinds of statements are known with certainty to be true because the very meaning of the terms involved, terms such as bachelors, triangles, things, wholes, requires that because we think of them in certain ways. In other words, we do not need to rely on sensory experience in order to "know" these things to be true. Consequently, we know about some things prior to sensory experience or a priori. Knowledge that comes after sensory experience is a posteriori.
Rene Descartes is another example of a rationalist. Instead of beginning philosophical inquiry with the study of the nature of reality, he suggests that we ask what it would mean to know about reality. To believe that reality is fundamentally water or some other "thing" is beside the point for Descartes unless we know first whether our belief itself is justified. To determine whether our beliefs are justified, we have to be able to trace them back to a statement, belief, or proposition that cannot be doubted. Descartes is searching for the one proposition upon which all other statements regarding truth can be founded.
Descartes arrives at this one basic proposition through a "suspension of belief" which has come to be known as "Cartesian Doubt." Certain individuals regard Descartes conclusions as bordering on the absurd, but the individual should understand that Descartes is only hypothetically or in a matter of speaking suspending all other beliefs in order to arrive at the one belief which he can "know" or attach himself to in order to reconstruct all other truth. In essence, through the use of Cartesian Doubt, Descartes is calling upon us to "pretend" that all is false with the exception of what has been called "The Cogito."
Descartes rejects knowledge based upon the senses or, as it has been called, a posteriori belief. For Descartes sensory experience cannot be regarded as true. One of the reasons for this conclusion on Descartes part is his speculation that we might be dreaming. In light of this possibility, we cannot be sure of the past, we cannot be sure of the external world, and we cannot be sure that other people have minds. In addition, Descartes also rejects mathematical truth, such as the proposition that 6 + 3 = 9. Neither can we trust the concept that all triangles have three sides since, in Descartes opinion, a demon may be deceiving us.
However, Descartes concludes that there is one truth of which we can be sure. We cannot be mistaken that we are thinking according to Descartes. From this idea emerged Descartes proposition, "Cogito ergo sum" or "I think therefore I am." "Thinking" proves that we exist. From this starting point, Descartes determines that he can begin constructing other truths; such as the truths of Identity and that things are composed of substances. For Descartes these qualities so to speak are innate since they are not determined by empirical experience.
However, Descartes was confronted with the possibility previously posed. What if he were being deceived by the so-called "evil genie"? Descartes does away with this possibility by positing the idea that an all-powerful, all good God would not permit this happening. Descartes attempts to prove the existence of God based upon the one thing that he can know for sure, the Cogito. Simply put, Descartes posits that he can posit the concept of imperfection and that the justification for this conclusion is based upon the existence of his doubts. Consequently, he must have some concept of "perfection" or else he would not know what "imperfection" is. He then concludes that he does not know what "perfection" is in and of himself. Rather he knows of "imperfection" because there must be a "God" who is perfect and thereby defines by his very being "perfection." Since God is perfect, he must also be all good and all knowing. As such he would never tolerate the existence of an evil genie, which would deceive. Therefore, God's existence, which has been proven for Descartes, disallows the possibility of the existence of an evil genie.
In short, for Descartes, certainty regarding other matters such as the existence of the external world is based upon knowledge of self.
