Ben Horowitz, the venture capitalist, is talking about the legends who watch over him in his Menlo Park, Calif., office. On one wall the stern faces of some of the forefathers of computer science -- the likes of Alan Turing and John von Neumann -- stare from inside a row of picture frames. Another wall is reserved for boxing greats: Ali, Frazier, and Foreman, James "Cinderella Man" Braddock, Sugar Ray Robinson, and a handful of others. "It's a little bit of a reminder to myself of what we are looking for in an entrepreneur," Horowitz says. "Great genius and great courage." The genius part is self-evident; without it there would be no Apple or Google or Microsoft. The courage part is less obvious. But Horowitz says no other quality -- not vision, not creativity, not charisma -- is more essential to succeed as an entrepreneur. Building a company is a lot like boxing, he says, and not because of any rush that comes from landing a metaphorical punch on your opponent. Building a company is hard and lonely. It demands relentless focus. And no matter how well you do, you must be ready to be pummeled again and again. "In boxing, you get hit, it's painful, then you sit on the stool when the adrenaline is gone and you feel that pain," he says. "And then you fight the next round."