Definition
Goodhart's law states that when a measure becomes a target, it ceases to be a good measure. The principle was first articulated by British economist Charles Goodhart in 1975 in the context of monetary policy: any statistical regularity will tend to collapse once pressure is placed on it for the purpose of control.
The popular form — "when a measure becomes a target, it ceases to be a good measure" — was coined by anthropologist Marilyn Strathern in 1997, restating Goodhart's original finding in a sharper, more general way.
Why it matters
How it works
Goodhart's original observation was about money supply: once a central bank announces a target, financial actors optimize around the target rather than around whatever the target was meant to indicate. The aggregate stops behaving like the underlying economy. Strathern generalized: any time a metric becomes the goal, people game it — sometimes consciously, sometimes structurally — and the metric loses its informational value.
The habit-design implication is real. A reading habit tracked by minutes-per-day can degrade into staring at a page while the timer runs. A writing habit tracked by words-per-day can produce filler that hits the count. A workout habit tracked by sessions can produce token gym visits that don't actually train anything. The form of the habit persists; the substance erodes. The tracker that was supposed to measure mastery is now measuring its own compliance.
Clear addresses this implicitly through reflection and review and through identity-based framing. Tracking is a tool, not the goal — the goal is becoming a certain kind of person, and the tracker is one piece of evidence among many. Periodically asking "is the measure still pointing at the thing I actually care about?" is what keeps the system honest. The discipline of measurement and the discipline of reviewing the measurement are different practices, and you need both.