I tend to use the metaphor of trying to carve the Grand Canyon with nothing more than water and gravity, but I appreciate you honoring and acknowledging the difficulty of this.

Honestly, I don't have formal criteria. I have young kids now, and I try to ask myself every once in a while, when they're old enough to really understand what I do for a living. When they ask me: "at that moment in history, what were you doing?" I want to be able to answer them and give them an answer that I and they will find satisfying.

I'll also be honest that I've my fill of conventional "success" in my life. I don't feel obligated to use the likelihood of success as a criterion anymore. I tend to just do the things that, in the moment, seem right to me. Or where it seems like, for whatever reason, no one else is willing or likely to try it, and I have been given that lonely assignment.

Many of the things that I've tried haven't worked out, but that's okay. I accept that that is a precondition of the kind of work I like to do.