First, Descartes noted that the testimony of the senses with respect to any particular judgment about the external world may turn out to be mistaken. (Med. I) Things are not always just as they seem at first glance (or at first hearing, etc.) to be. But then, Descartes argues, it is prudent never wholly to trust in the truth of what we perceive. In ordinary life, of course, we adjust for mistaken perceptions by reference to correct perceptions. But since we cannot be sure at first which cases are veridical and which are not, it is possible (if not always feasible) to doubt any particular bit of apparent sensory knowledge.
The Dream Problem
Since my most vivid dreams are internally indistinguishible from waking experience, he argued, it is possible that everything I now “perceive” to be part of the physical world outside me is in fact nothing more than a fanciful fabrication of my own imagination. On this supposition, it is possible to doubt that any physical thing really exists, that there is an external world at all. (Med. I)
A Deceiving God
Descartes means to raise the far more devastating possibility that whenever I believe anything, even if it has always been true up until now, a truly omnipotent deceiver could at that very moment choose to change the world so as to render my belief false. On this supposition, it seems possible to doubt the truth of absolutely anything I might come to believe.