Contrary to contemporary thought a skeptic need not necessarily be a negative person. In fact Rene Descartes believes a negative thinking person is the polar opposite of a skeptic, opinions expressed in the negative would lead Descartes to dismiss such person from being a legitimate skeptic. Descartes believed that to be a skeptic one needs to doubt all that he or she knows or understands. Upon this doubt he believes one can build a path to knowledge. This foundation of knowledge requires doubt because without it you cannot eliminate error. Descartes believed that every day we cultivate opinions that were strewn with errors, and all these errors built upon each other made our opinions "unreliable".
Descartes contends that our senses lead to unreliable conclusions and opinions. For instance a straight stick partly in water appears to be bent. This he says is an illusion filled with errors, so the only way to rid ourselves of error is to doubt as a skeptic. His straight stick example points out the value of meditation and reflective thought since all that appears to our senses may be unreliable perception. .
What Descartes suggests in his works can scare the every day man because he is asking us to take nothing for granted. He is asking us to shed all our prejudices and preconceived notions, and only then can you build a base of reliable knowledge. Descartes himself described Meditations as being a philosophy much like a tree. "The roots of the tree are metaphysics, the trunk is physics, and the branches all the other sciences".
In Meditation One Descartes flooded his mind with doubt. On page 17 of Meditations on First Philosophy Descartes says " it was necessary. to demolish everything completely and start again right from the foundations if I wanted to establish anything at all in the sciences that was stable and likely to last.".
Descartes couldn't spend all day disproving every opinion he ever had so he suggested a huge leap of understanding from the start.