What does it mean to be rational in a world of radical uncertainty? Once we are liberated from the view that there is a single optimising solution, rules of thumb--technically known as heuristics--are better seen as rational ways to cope with an unknowable future. A heuristic is a decision rule that deliberately ignores information. It does so not just because humans are not computers, but because it is rational to ignore information when we do not understand how the world works. [...] Heuristics are not deviations from the true optimal solution but essential parts of a toolkit to cope with the unknown.