Milton Friedman's Thermostat

mental-model

When a controller actively holds an outcome steady, ordinary correlations can hide—or appear to reverse—the forces acting on it. Stability may be evidence of continuous intervention, not causal independence.

A car stays at 22°C whether the road outside is cool or scorching. The data might therefore suggest that cabin temperature has nothing to do with outside temperature—or with diesel use. In reality, that missing correlation is the air conditioner succeeding.

The controller erases the visible relationship

Three moving parts matter: an outside disturbance, a target value, and a controller that spends effort correcting deviations from that target. As outside temperature rises, the air conditioner works harder to keep the cabin at 22°C. The disturbance changes the required intervention while the measured outcome barely moves. If you examine only outside and inside temperatures, the causal pressure disappears from view; the compensating diesel use is where it has gone. This is a special trap within Correlation vs Causation: weak association can coexist with a strong Causal Mechanism when feedback suppresses the variation you expected to observe.

Where it shows up

The 22°C cabin

The stable cabin temperature does not show that outdoor heat is irrelevant. It shows that the air conditioner is continually cancelling its effect, with the extra work appearing in diesel consumption rather than cabin temperature.

Stability alone proves nothing

A flat outcome is not automatically evidence of a hidden controller. This model applies only when you can identify a target, a disturbance, and a compensating process linking them. Without those moving parts, invoking feedback merely replaces one unsupported causal story with another.

Measure the correction, not just the outcome

When an important variable looks strangely stable, identify what is holding it there and track that system’s effort. In the car, compare outside temperature with air-conditioning load or diesel use—not only with the cabin’s regulated 22°C.

Episodes that teach this

The Great LIE - Correlation is Not Causation - FutureIQ · explained at 13:03 7,309 views
Car AC keeps inside temperature at 22 degrees, so inside temperature appears unrelated to outside temperature and diesel usage.