A well-known joke tells of the economist in the wilderness who, when he sees a bear approaching, pulls out his computer and begins to calculate an optimal strategy. His colleague, appalled, says: ‘We don’t have time for that!’ ‘Don’t worry,’ replies the economist smugly, ‘the bear has to work out an optimal strategy too.’ Behind the joke lies a deeply serious point. The bear gains a decisive advantage by not suffering the illusion that the approach based on calculation might work.