The inverse of Chesterton's Fence is this:

Sometimes a reformer comes up to a spot where there once was a fence, which has since been torn down. They declare that all our problems started when the fence was removed, that they can't see any reason why we removed it, and that what we need to do is to RETVRN to the fence.

By the same logic as Chesterton, we can say: If you don't know why the fence was torn down, then you certainly can't just put it back up. The fence was torn down for a reason. Go learn what problems the fence caused; understand why people thought we'd be better off without that particular fence. Then, maybe we can rebuild the fence—or a hedgerow, or a chalk line, or a stone wall, or just a sign that says “Please Do Not Walk on the Grass,” or whatever it is that we really need.