The Chesterton fence is the idea that you come across a fence in the middle of a field and think, well, this looks just like it gets in the way. And when you tear it down, you find out it had a purpose, but it doesn't make sense to you. Many heuristics, many rules of thumb are like that; they've evolved over time. They're consistent with the way our brain works. Yet as arrogant experts, we often say, oh,Well, this just must be a mistake. We change policy or make decisions accordingly.