Rationalism is the idea that reason alone is a source of knowledge and is separated from experience. Rationalism allows Descartes to think of all reasonable doubts and to find certainties. In Meditations One, his skeptical argument was that nothing must be real because everything is made by the same deceiving god, but with reasoning he can gain a certainty from that.
Descartes rational side having truly and so “thoroughly thinking the matter through [he] conclude[s]... this proposition, [he] is, [he] exist[s],must be true whenever [he] assert it or think it,” (4). To think he must exists and that can be certain. Even though the content he thinks about may be deceiving, he is still able to think making him real. This is a display of rationalism because despite what his senses were telling him he was able come to one absolute truth. Descartes was able to meet his goal to find at least once certainty with the help of