Warren Buffett in the summer of 1998 (from the brilliant joint interview with Bill Gates that I linked to the other day):
They’ve asked us to start out talking, the two of us, about what got us here, but then it’s on to your questions. How I got here is pretty simple in my case. It’s not IQ, I’m sure you’ll be glad to hear. The big thing is rationality. I always look at IQ and talent as representing the horsepower of the motor, but that the output—the efficiency with which that motor works—depends on rationality. A lot of people start out with 400-horsepower motors but only get a hundred horsepower of output. It’s way better to have a 200-horsepower motor and get it all into output.
So why do smart people do things that interfere with getting the output they’re entitled to? It gets into the habits and character and temperament, and behaving in a rational manner. Not getting in your own way. As I said, everybody here has the ability absolutely to do anything I do and much beyond. Some of you will, and some of you won’t. For the ones who won’t, it will be because you get in your own way, not because the world doesn’t allow you.
This point is almost seems too obvious and yet at the same time, too subtle. Both of which are undoubtedly why such a key aspect of life is often overlooked when talking about why someone achieves greatness.