It's incredibly hard to identify successful companies early on. But following Thiel's rule isn't actually all that hard.

The key is to look at the total addressible market for a company or product. It has to be large, or growing quickly, or both. For example, AirBnB might have looked like a bad idea, but the hospitality market is huge, so if it did work out then they could grow to become a giant company. On the other hand, you could create a transformative product for blind people, and build a business around it that makes you a multi-millionaire. But VCs will never invest in your company because there just isn't a big enough market. You might 3x or 5x an investment but you'll never deliver the kinds of giant returns VCs need in order to make their LPs happy.

Of course, I'm talking about traditional VC firms - like the one the blog post's author runs. There are all kinds of investors out there with different motivations.