Chesterton's Fence cautions us against making changes rashly, before we understand the reason why something is the way it is.
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.
The Onion In The Varnish cautions us against accepting the status quo of how things are done. Ingredients we don’t understand should prompt us to ask what purpose they’re really serving.
In The Periodic Table, Primo Levi tells a story that happened when he was working in a varnish factory. He was a chemist, and he was fascinated by the fact that the varnish recipe included a raw onion. What could it be for? No one knew; it was just part of the recipe. So he investigated, and eventually discovered that they had started throwing the onion in years ago to test the temperature of the varnish: if it was hot enough, the onion would fry.
Chesterton’s Fence and The Onion in the Varnish seem obviously in conflict, right? Chesterton pushes us to conserve that which we don’t understand. Onion encourages questioning the need for that which we don’t understand. Chesterton in the varnish factory would keep throwing in the onion as long as he doesn’t understand its purpose. How to tell which is right and appropriate to apply in a given situation?
But really, both anecdotes teach us the same thing: it’s important to understand why things are the way they are. Both tearing down the fence and continuing to throw in the onion are bad, if you don’t understand why they’re being done.
This is similar to the lessons in The Secret Of Our Success and Seeing Like A State. Both encourage a healthy respect for culture. It’s easy to dismiss practices that seem silly, but the obvious / “correct” alternative may introduce unintended consequences. (On Chesterton's Fence excerpts excellent examples from both books.)
To summarize: the world is complicated, there may be unintended consequences, understand before you act.