Interesting question. Richard Feynman always used to tell me that he thought “peace of mind” was a critical prerequisite to doing creative work. But certainly not everyone in the book had that when they were doing their most important work. And actually I think if there’s one theme I noticed it’s that external stimuli and external constraints often seem to play a crucial role. People don’t seem to come up with great ideas by just sitting around and thinking for a long time. What seems much more often to happen is that there’s some external question or external issue that comes up—or the people are just trying to do some job they have—and thinking deeply about that is what leads them to do some great idea.