Goodhart's law: any measure good enough to steer by becomes, the moment you steer by it, a target — and a target is gamed, not met. The number and the thing it stood for quietly divorce, and the number is the one that keeps showing up to work.
I. The divorce
A proxy is chosen because it tracks something real and hard to see. Then it is made the goal. Pressure flows to whatever is watched, and what is watched is the proxy.
II. The map that kills the forest
The deeper version belongs to James Scott. To manage anything at scale you must first make it legible — count it, flatten it, fit it to a grid. But the act of making a thing legible strips out exactly the local, tacit, unwritten detail that made it work.
His example is literal. The scientific forest of eighteenth-century Prussia was planted in tidy rows so it could be measured and sold — and within a generation it was dying, because the forest had never been the rows of marketable timber the ledger could see. It was the undergrowth, the fungus, the rot, the mess the ledger left out. The map was drawn to control the forest, and it killed it by mistaking itself for the thing.
III. The cost it returns
The reflex is to measure better — to hunt for the un-gameable metric. There isn't one. The fault is not the choice of measure; it is the substitution. The instant a proxy carries the full weight of the real thing, the pressure finds it, and the real thing is quietly set down.
You cannot out-design Goodhart. You can only refuse to let the number become the goal — keep it a symptom you read, never a target you chase. Which asks for the one thing the measure was meant to spare you: judgment. We reach for numbers to escape the burden of judging, and the numbers, the moment we trust them, hand it back with interest.