Knowledge is not ultimately sense knowledge

According to philosopher Rene Descartes, whatever is clearly and distinctly perceived is true is the best general rule of knowledge. Descartes begins his meditations by reflecting his previously held beliefs have turned out to be false, and worked to receive indubitable knowledge. This is the process of methodological doubt. In order to rid hilself of false opinions, he must immediately reject any beliefs that is subject to doubt, even if that belief is likely to be true.

Any beliefs are subject to doubt. We cannot trust the senses because we can never be completely certain that we are not dreaming. Descartes imagines that an evil demon of great power has employed all his energies to the purpose of deceiving him. The heavens and the earth, even his own body or any corporeal object will consider a mere illusion brought about by the influence of this demon.  Regarding his own existence, he has already denied that he has a body and senses, but here he pauses. Even without body and sense, Descartes says, he must still be something. Now he knows that, he exists, and he exists as a thing which thinks.  As a thinking being, Descartes also possesses the power of the many attributes of thought; affirming, denying, willing, refusing, doubting and understanding among them.

According to him, an isolated ego, the ego itself merely another illusion. Yet this oen indubitable proposition is hardly remarkable when examined closely; all that can be proven by thinking is the existence of thinking itself.