Goodhart's Fallacy

Goodhart’s law ⁠ is a somewhat famous statement, essentially saying that “when a measure becomes a target, it ceases to be a good measure”. I’ve thought that this naturally implies a construction as a fallacy, and just wanted to write it down. Goodhart’s fallacy, I posit, is the fallacy of appealing to a measure without verifying its capabilities of measuring your target. A simple example I…