March 30, 2020 — Events like the COVID-19 pandemic of 2020, the US housing crash of 2007-09, and the terrorist attack of September 11, 2001, are called “black swans”: in each case, few people were able to predict them reliably, at least not with precision. But they were known unknowns, not unknown unknowns. That is, in each case, knowledgeable analysts were fully aware that such a thing could happen, even that it was likely to happen eventually. They could not predict that the event would happen with high probability in any given year. But the consequences of each of these events were severe, and predictably so. Thus, policymakers should have listened to the warnings and should have taken steps in advance. They could have helped avert or mitigate disaster if they had done so.