Descartes is famed by is familiar notion, “I think therefore I am (Cogito, ergo sum.).” It is a conclusion he has reached in his second meditation after much deliberation on the existence of anything certain. After he discovers his ability to doubt and to understand , he is able to substantiate his necessary existence as a consequence. What we doubt or understand may not ultimately correspond, but we can never be uncertain that we are in the process of thought. This idea is a major component in Descartes’ proof of the external world. He relies on the existence of a non-deceiving God to ensure that an external world exists after calling it into doubt