JARED DIAMOND: In a crisis, both a personal crisis and a national crisis, there's the issue that's called building the wall, which like many things, can be healthy or unhealthy. When we have a personal crisis, for example, a marital crisis or a career crisis, often we feel everything in my life has gone wrong. I'm overwhelmed. My life is in a total mess. And when you feel that way, there's no way that you can attack the problem, because you feel that everything is messed up. You have to build a wall, and you have to delineate -- within the wall is the thing: Your life has gone wrong. You messed up your marriage. But outside that wall, your relationships with your friends and your job, they're perfectly OK.

Similarly with nations -- nations, when they encounter a crisis, they have to build a wall -- in a good sense. They have to recognize what is not working and recognize what is working. The United States has problems today. But there are wonderful things about the United States. We have a long history of democracy. We have a federal system, which is a great system of government. We profit from this wonderful geography. We've been able to use immigration throughout our history creatively, more creatively than any other country that I know of. And so, outside the wall are all these things that are working well in the United States. Inside the wall, we've got problems. We should not feel overwhelmed with a sense that everything is messed up with the United States. No, it's not that messed up.

That's a good form of isolation, building a wall. A bad form of building a wall is cutting yourself off from the outside world. That's no longer possible for the United States or any other first world country, because in this globalized world, they, out there, can do things. They can reach us. They can send immigrants. They can send terrorists, unintentionally, diseases spreading from tropical countries can reach temperate zone countries. In the 1920s and 1930s, the United States had an isolationist foreign policy. And that meant postponing the day of reckoning when we had to deal with Germany and Japan. In short, isolation can be harmful. But isolation is also necessary, isolating what works from what doesn't work.