To Identify Greatness
This is one of the best paragraphs I've read in a long time. Provided by Paul Graham.
Because you can't tell a great hacker except by working with him, hackers themselves can't tell how good they are. This is true to a degree in most fields. I've found that people who are great at something are not so much convinced of their own greatness as mystified at why everyone else seems so incompetent.