What creates civilization, in the most basic sense? According to Professor David Frye, who joins me this week, it's walls - Hadrian's Wall, the Great Wall of China, city walls, and thousands more over the millennia.Support this show by supporting our sponsors!Linzess - Learn about ways to make your life a little happier. Visit OhMyGut.info/podcast.Indochino - Get Patrick's favorite suit for 50% off at Indochino.com and enter code TIDES at checkout.Lightstream - Apply now to get a special interest rate discount at Lightstream.com/TIDES