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 DescartesÕ 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 by the invocation of the dream argument. In this argument, Descartes suggests the possibility that none of our ideas are caused by external objects and therefore, such objects may not exist. He also raises the idea of a demon that may deceive us and allow us to perceive what is not really there. Although he assures himself of his own existence by his modes of thought, he remains uncertain of the reality of an external world. He doubts whether there is anything of material substance that provokes thought within him rather than it being conceived in his mind completely independent of anything else. Descartes then considers those reasons that have inclined him to believe these material things exist in the past. "I know by experience that these ideas do not depend upon my will, nor consequently upon myself, for often I notice them against my will... I feel heat, and therefore I believe that this feeling or idea of heat comes to me from something other than myself, namely from the fire I am near. Nothing is more obvious than the judgment that this object (rather than something else) grafts its likeness on to me." Since however, he has called upon anything to be false that provokes any doubt he does not believe this explanation to be enough for the proof of the external world. Relying on this sort of natural impulse has led him astray in the past, so what is to keep it from happening all the time. He also calls upon the dream argument in this instance. Ideas come to be in dreams independent of external objects and perhaps this is true of ideas when we are awake. It seems that Descartes finds it necessary to first establish the existence of a non-deceiving God before he can be assured of the existence of anything beyond himself and his mode of thought. He does this by the rationalization that his perception of God is that of a perfect being. In order for a being to be perfect it must exist. Since he himself is an imperfect being, he can not conceive the idea of perfection on his own. Therefore, it must have come from some other faculty that must be perfect, which is God. It is after his proof of the existence of God that Descartes comes to accept that clear and distinct ideas can be trusted. After this deliberation his process of coming to the existence of an external world seems rather direct. I have the clear perception that material objects exist. Since I have already determined that God is not deceiving me nor my perceptions, my perceptions can be trusted as being actual. If my perceptions of material objects and an external world are actual, then they must exist. But this external reality is different from our reality of thought. It becomes dualistic by the idea of two separate substances. Descartes establishes a sort of isomorphic state between thought, or the mind, and extension, or matter. The mind takes up no space. It consists of the senses and all modes of thought. Matter, however, takes up space. It can be divided into smaller and smaller components, but it, unlike the mind, has no consciousness. Both, he infers have God as their source because God, alone, exists independent of anything else. But, the substances do not have any contact with each other. Thought is independent of matter just as matter is independent of thought. But, although the two are indeed completely independent of each other, there is constant interaction between the two. This is the essence