“If I, today, imagine how I’d respond to stocks falling 30 percent, I picture a world where everything is like it is today except stock valuations, which are 30 percent cheaper. But that’s not how the world works. Downturns don’t happen in isolation. The reason stocks might fall 30 percent is because big groups of people, companies, and politicians screwed something up, and their screwups might sap my confidence in our ability to recover.”