Cartesian skepticism has led Descartes to doubt his senses to the highest extent. If he is seeing something far, he could be mistaken - he could be perceiving a metaphorical shadow on a cave wall. And even if he isnât he could be having the most vivid, but inaccurate, dream.
He attempts to resolve this by appealing to God. As He would surely not have created a world where human beings would not be systematically deceived. However this is also doubted. As some evil demon bent on deceiving and confusing us may exist instead. Either way, Descartes rationalizes that: Because he can know that he thinks, he knows he may be deceived, because he thinks, he rationalizes that he must exist. That there has to be some object to deceive.
Human brains create a functional but stimulated version of reality. Cartesian doubt argues that perception is taken for granted. Perception could be illusion, or dreamlike thereby rendering out perception of reality obsolete.