Chesterton's fence

The line of code

A developer cleaning up an old program finds a line that waits half a second before saving a file. “This just slows everything down. Nobody knows why it’s here,” she says, and deletes it. A week later, files saved to the shared network drive start coming out corrupted.

Not knowing why the line was there was treated as knowing it had no purpose. A few minutes spent reading the history of that line, or asking the person who wrote it, might have turned up the reason: the network drive needed time to release a lock on the file. The principle doesn’t say the delay had to stay forever. It says to find that out before deleting it, and then decide whether there’s a better fix.

The rule nobody remembers

A new office manager notices that every purchase over $500 needs two signatures. “It’s pure red tape,” she says. Before scrapping it, she asks the finance team. They tell her it was added years ago, after a vendor was paid twice for the same invoice.

Here the fence is understood, and understanding it changes the question. It’s no longer “is this rule pointless?” but “is a second signature still the best way to catch duplicate payments?” If the accounting software now flags duplicates automatically, she has a good reason to drop the rule, and it’s a different reason from “I didn’t see the point.”

The reason that expired

A family always runs the dishwasher after 10 p.m. Asked why, one of the parents remembers: years ago, their electricity plan charged much less for power used overnight. They’ve since switched to a flat-rate plan.

This is the fence doing what Chesterton said it should. Once the reason is known, it can be judged, and this one no longer applies. The family can run the dishwasher whenever it suits them. The principle asked them to look, not to keep the habit.

The principle comes from the English writer G. K. Chesterton’s book The Thing (1929), at the opening of an essay, “The Drift from Domesticity”, which goes on to apply it to family life. The name “Chesterton’s fence” came later. His own statement of it:

In the matter of reforming things, as distinct from deforming them, there is one plain and simple principle; a principle which will probably be called a paradox. There exists in such a case a certain institution or law; let us say, for the sake of simplicity, a fence or gate erected across a road. The more modern type of reformer goes gaily up to it and says, “I don’t see the use of this; let us clear it away.” To which the more intelligent type of reformer will do well to answer: “If you don’t see the use of it, I certainly won’t let you clear it away. Go away and think. Then, when you can come back and tell me that you do see the use of it, I may allow you to destroy it.”

His reasoning is that the fence “did not grow there”: “Some person had some reason for thinking it would be a good thing for somebody. And until we know what the reason was, we really cannot judge whether the reason was reasonable.” If something set up by people like ourselves looks completely meaningless, he adds, “it is extremely probable that we have overlooked some whole aspect of the question.”

The passage that matters most for telling the principle apart from a defense of tradition comes next. Someone who knows how the institution arose, “and what purposes it was supposed to serve”, Chesterton writes, “may really be able to say that they were bad purposes, or that they have since become bad purposes, or that they are purposes which are no longer served.” Understanding the fence is the condition for removing it, not a reason to keep it. He also meant it to apply widely, “to trifles as well as true institutions, to convention as well as to conviction.”

Chesterton’s fence sits between two fallacies about old things, and it’s easily mistaken for one of them.

• It isn’t the appeal to tradition. The appeal to tradition says a rule is right because it’s old or has always been followed. The fence says only that the rule probably had a reason, and that you should learn the reason before judging. Chesterton explicitly allows that the reason may turn out to be bad, or no longer relevant. “We’ve always done it this way” is a conclusion; “find out why we’ve done it this way” is a question.
• It isn’t Chronological snobbery either, but it’s the direct counterweight to it. Chronological snobbery dismisses a practice for being old or out of fashion. The fence refuses that shortcut, much as the test that page takes from C. S. Lewis does (was it ever refuted, or did it merely go out of fashion?): what matters is what the thing was for and whether that still holds, not when it started. The appeal to novelty, replacing something simply because the alternative is newer, skips the same step.
• It targets an argument from ignorance. Chesterton’s “modern type of reformer” reasons from “I don’t see the use of this” to “it has no use.” Not having found a reason counts against there being one only if you’ve actually looked where the reason would be.
• Knowing the origin isn’t the verdict. The fence asks where a rule came from, but deciding its fate by its origin alone would be the genetic fallacy. A rule set up for a foolish reason can still be useful now, and one set up for a good reason may no longer serve it. The origin tells you what to check, not what to conclude.
• It isn’t the sunk cost fallacy. Keeping a fence because it was expensive to build looks backward at what was spent; Chesterton’s fence looks at what the fence does.

The practical question is “What was this for, and is that still true?” Look for the history (old notes, the version history of the code, the person who set it up), and ask what would happen if the fence weren’t there. If the answer is “nothing we still care about”, you can take it down, and now you can say why.

• The reason may be unknowable. The records may be gone and the people who set it up may have left. At that point “find out first” can’t be satisfied, and you have to decide on other grounds: what the fence costs, what could go wrong without it, and whether you can remove it in a small, reversible way and watch what happens.
• Investigating has a cost too. Chesterton meant the principle to cover trifles as well as institutions, but the effort worth spending should scale with the stakes. Tracing the history of a harmless habit can cost more than simply trying life without it, especially when the change is easy to undo. A fence guarding something dangerous deserves more patience than one guarding a preference.
• Not every fence had a reason. Chesterton’s premise is that someone “had some reason”. That’s often true, but some rules and habits arise by accident or by imitation, and some were based on a mistake from the start: a line of code pasted in by accident was never for anything. Asking “why is this here?” is still worth it, but the answer may be “no reason.”
• Having a reason isn’t having a good one. A rule can last because it serves whoever made it rather than the people it’s imposed on. The philosopher Ibo van de Poel, discussing a version of the principle, points out that norms can survive for many reasons other than serving shared ends, including being “functional to the powerful”.
• Delay has costs. Van de Poel also notes that while the principle protects against changing things too easily, there are situations where “the bigger danger is in not changing entrenched norms” fast enough to deal with a new problem. Leaving a fence in place while you investigate is itself a choice, with its own risks.
• It can be used to stall indefinitely. If no account of a fence’s purpose is ever accepted as good enough, or the standard for “understanding” keeps rising, the principle has stopped being a request for reasons and become a way of never changing anything. Chesterton’s own version ends with permission: come back when you see the use of it, and “I may allow you to destroy it.”

So “you don’t know why it’s there” is a good reason to look before removing something, not a verdict that it must stay.