What is Greatness?

What is greatness? Two different portraits of two different successful men (basketball superstar Kobe Bryant and Star Trek: The Next Generation mastermind Michael Piller) show some surprising similarities:
http://www.probasketballnews.com/celestand_1110.html
http://blog.scifi.com/battlestar/archives/2005/11/index.html#000091

The great have an uncanny degree of self-confidence, a willingness to ignore the conventional wisdom and what others think in order to pursue their own path. I’ve noticed the same thing in many of the CEOs and entrepreneurs that I’ve worked with.

Of course self-confidence without justification is arrogance and hubris. The trick is knowing whether or not your belief in yourself is rational. Ultimately, that has to come from within–the outside world will call you a dreamer and a fool up to the point where you succeed, at which point you’ll be hailed as a visionary.