Certainty is a good thing, up to a point. When looking through a marketing magazine once, I saw an advertisement with the maxim Never Doubt Your Beliefs, and Never Believe Your Doubts. It's excellent advice for salesmen who meet tough resistance from customers every day, but I don't want a person who adheres to this maxim to be in charge of deciding when to put a new machine into service.

The contemporary insult "clueless" is directed at people we see as completely out of touch with important events. If you adapt that word a little, we come up with the new insult of "doubtless", meaning a person who is rock-solid certain he knows all he needs to know and believes that nothing can go wrong. Being doubtless is as dangerous in its own way as being clueless.