In the new book Alpha Girls, Julian Guthrie writes about Reid Dennis, the founder of IVP, a venture capital firm with over $7 billion in committed capital:

Even during the darkest days of the dot-com crash, when the small team hunkered down and pondered whether the world as they knew it was ending, Reid Dennis remained positive.

One of his favortie sayings was, "I don't know many rich pessimists."

That's because it's definitely possible to succeed without capital. Or without a business plan. Or without a marketing plan. Or even without a great idea.

But the one thing you must possess in order to be successful?

Irrational optimism.

That means finding a way to ignore your doubts: That you aren't smart enough, or dedicated enough, or adaptable enough, or simply that, in spite of your best intentions and your very best effort, you won't succeed.

And finding a way to ignore the doubts of others. Family and friends tend to shoot multiple holes in your ideas, not because they want to bring you down but because they care about you and don't want to see you fail. That's why people rarely say, "Hey, that's a great idea. You should go for it!"  Most people aren't wired that way. Most people are much better at identifying and describing potential problems. 

And that's why, if you want to be successful, you need to be irrationally optimistic: Not because the odds are stacked against success, but because irrational optimism helps you succeed in ways capital, business plans, and marketing savvy can't.

Of course, you can take irrational optimism too far.

But then again, maybe you can't.

Think about sports, the ultimate zero-sum game. Only one individual or one team can win, but great athletes still go into every game believing they will win -- because if they don't believe they can win, they've already lost.

Is complete self-belief irrational? Sure. Is it also a requirement for high-level athletic success? Absolutely. Great athletes push aside doubt and disbelief.

So do great entrepreneurs.

If you listen to the naysayers you'll never start a business, never expand, never work and struggle and overcome -- and never succeed. If you don't believe in yourself, however irrationally, you will not succeed.

Although no amount of self-belief is enough to ensure success, the smallest bit of doubt can ruin your chances.

In Bounce, Matthew Syed quotes then-Arsenal manager Arsene Wenger, whose 22-year career made him the most successful manager in Arsenal history and one of the most successful managers in the English Premier League, on a great athlete's perspective:

"To perform to your maximum you have to teach yourself to believe with an intensity that goes way beyond logical justification.

"No top performer has lacked this capacity for irrational optimism; no sportsman has played to his potential without the ability to remove doubt from his mind."

The same is true for entrepreneurs.

And, really, for everyone.

Be smart, be logical, be rational and calculating, and never stop trying to improve your skills.

But most important, be irrationally optimistic.

Because believing in yourself will take you to places no external forces ever can.