My method of approaching the issue is to divide Doubt into two categories: non-philosophical doubt and philosophical doubt.
Non-philosophical doubt is based primarily on an obvious prejudice or assumption. For example, a staunch conservative might doubt that Barack Obama would make a good president--because he or she assumes
that Obama's policies would ruin the nation. An atheist might doubt that the existence of God can be proven--because he or she firmly believes
that God is a fiction. The dogmatic skeptic might doubt that Truth is ever attainable--because he or she doggedly insists
that knowledge of anything is impossible. And most of us might doubt that the sun is the center of the universe--because we prudently assent
to the proclamations of modern physics and astronomy.
On the other hand, philosophical doubt is based more on open-minded curiosity than on any obvious prejudice. In contrast to the non-philosopher, the philosopher simply will wonder
what makes a good president; will try to figure out
whether God really exists; will reflect upon
the nature of knowledge and truth; and will even dare to imagine
that the Soul is the center and circumference of all being.
Curiosity, the anticipation of discovery, and the love of wisdom are the driving force behind the philosopher's doubt; while the non-philosopher's so-called "doubt" is mostly the byproduct of an unphilosophical belief, an indirect manifestation of some deep-seated prejudice combined with an irrational love of one's own.
As would-be philosophers, we should always watch our thoughts to see what passions motivate them. This may be the hardest task of all: To know thyself.