Mr. Donald Trump talking about the US Mexican border, he's not doing anything new or unusual. It's something that's very much in the air and perhaps always has been. The Great Wall of China has been there for a great long time. People have always tried to keep foreigners out. There is a very natural desire, once you've got somewhere cozy, to keep it to yourself, and equally there's a very strong impulse for those who are not at such a safe and prosperous place to try and get in. It's been creating conflict throughout human history.