This is one of my favorites!  From the Book of Qualities:

Doubt

Doubt is a little imp who sits in the back of your mind, always just out of your line of sight.  doubt waits patiently until a comment is made--usually in jest--then rushes forward to stand tall, arms wide, ready to leap in to the abyss, and take you with him.  He likes to stay there for as long as you like.  Doubt travels with you, though sometimes you don't notice him.  He can jump from mind to mind and replicate himself like a cell.  He is a parasite who feeds on your past and regurgitates your future, never minding the present.  Sometimes, Doubt looks out through your eyes at the world around you.  Doubt likes to throw big parties sometimes.  his favorite friends are Fear, Uncertainty, and Worry.  the four of them stay up late into the night creating instead of solving the world's problems.  Doubt and his friends also like to argue sometimes with Faith, Trust, and Confidence.  When they show up, Doubt shrinks back until eventually he and his friends move on.