If anyone thinks they'd rather be in a different part of history, they're probably not a very good student of history. Life sucked in the old days. People knew very little, and you were likely to die at a young age of some horrible disease. You'd probably have no teeth by now. It would be particularly awful if you were a woman.

I wouldn't say I have a lack of fear. In fact, I'd like my fear emotion to be less because it's very distracting and fries my nervous system.

And we need things in life that are exciting and inspiring. It can't just be about solving some awful problem. There have to be reasons to get up in the morning.

For me it was never about money, but solving problems for the future of humanity.

Going from PayPal, I thought: 'Well, what are some of the other problems that are likely to most affect the future of humanity?' Not from the perspective, 'What's the best way to make money?'

I realized that a methane-oxygen rocket engine could achieve a specific impulse greater than 380.

The United States is definitely ahead in culture of innovation. If someone wants to accomplish great things, there is no better place than the U.S.