Airbnb is worth something in the neighborhood of $30 billion now. But back in 2008, the founders had their sights set significantly lower. As co-founder Brian Chesky explained on Medium, they were attempting to raise $150,000 at a $1.5-million valuation. That means you could have bought 10 percent of the company for all of $150,000. 

But when Chesky and his co-founders approached seven prominent Silicon Valley investors, five passed and two ignored them completely. 

There are a couple lessons to take from this story. From the entrepreneur's perspective, it's an illustration of the importance of perseverance. From the investors', it shows just how hard it is to spot a billionaire in the making

A home rental platform may sound like a no-brainer business plan now, but back when Chesky was pitching paying to crash on a stranger's air mattress, the idea just seemed insane. How do the best investors spot the difference between make-you-a-billion-dollars crazy and just plain crazy? Investor Jason Calacanis, who made early (and lucrative) bets on Uber, Calm, Trello, and many other companies offers a simple formula. 

"Delusional, but with skill." 

The insight comes from a long interview Calacanis did for The Jordan Harbinger Show, helpfully highlighted on Medium by Alan Trapulionis. In the course of the conversation, Calacanis admits he was one of those folks who thought Airbnb was a terrible idea at the beginning. But when Calacanis gets it right, it's because he finds a founder who is "delusional, but with skill."

Like every other commentator on entrepreneurship, Calacanis stresses the importance of resilience and indefatigability. But he also stresses the importance of betting on long odds to get high returns. To do that, you need to bet on ideas that look crazy. What's another word for people who believe in crazy ideas? Delusional. 

But obviously, being willing to take huge risks isn't a guaranteed path to riches and success. More often than not, that alone ends in disaster. So to spot a potential unicorn founder, you need to look for delusion paired with exceptional craftsmanship. 

"I'm looking for somebody who is delusional, with skill," he said to initial laughs from the audience. But he's serious. Calacanis uses Elon Musk's initial pitch to go to Russia, learn about rockets, and then come back and make some in the States as an example. 

By all traditional measures, sincerely believing you could build SpaceX is almost certifiably insane, but what keeps Musk and other founders on the right side of the line separating sanity from lunacy is "craftsmanship and the detail work," according to Calacanis. He tells the story of when Travis Kalanick pitched him Uber, as an example.

"There were two taxis in San Francisco at the time and the taxi was going down the road sideways. And I was like, 'What is that guy, Tokyo drifting? Why is it like that?' And he was like, 'Oh, it's a really hard problem, Jason. The iPhone 3G just got GPS and the car's coming down, but there's latency and what we're going to do is, we're going to make seven or eight versions of the car and then ..." and on and on Kalanick went. 

That maniacal attention to detail and deep knowledge of the space is the essential complement to huge dreams. Without one, a company won't have the scale to generate massive returns. Without the other, the founder won't have the craftsmanship to make a product that delights customers. 

Do you check both boxes? 

The entire entertaining interview is worth watching (check it out below), but this bit of it isn't just helpful for investors looking to pick big winners before anyone else. It's also a handy cheat sheet for would-be entrepreneurs. If you want to create something world-changing, make sure you check both of Calacanis's boxes. 

Your idea must be both so outlandish-sounding that most people think you're crazy, and you must be sufficiently obsessed with executing on it that you can get deep into the weeds on even the smallest details. If you're not delusional with exceptional craftsmanship, you're unlikely to build a billion-dollar business.