Scott Sumner writes : "Bubbles are caused by asset prices crashes, just as mountains are caused by valleys." Sumner's contention is thatwe only see "bubbles" when NGDP crashes, and leads to a steep price decline in the asset class that we then declare to have been in a bubble. Well, first of all, mountains aren't caused by valleys! The land surface of the Earth would not be a 29,000 plateau if not for those pesky valleys: tectonic plates collide, and lift up mountains. So the metaphor is bad, but what about the economic analysis? Sumner says: "The 1920s and the Great Moderation both saw relatively stable NGDP growth. So why the big bubbles? Because NGDP growth crashed in 1929-30 and 2008-09. In 2008-09 NGDP growth slowed by 9% relative to trend, nothing like that happened in the 1970s. It was the crash that (partly) created the bubble. Without the crashes, we wouldn’t even be talking about the great stock bubble of 1929, or the great housing bu...