A driver on a familiar commute turns left at the intersection without thinking. The turn is triggered by the visual cue, not by a mental simulation of the route. This is model-free control: fast, automatic, blind to change. If the usual road is closed, the system will still try to turn left.

A driver encountering the closure stops, considers alternatives, mentally traces each route, and chooses. This is model-based control: slower, effortful, flexible.

The brain runs both. The dorsal striatum drives habit. The prefrontal cortex and hippocampus simulate alternatives. Which dominates depends on the situation.

Stress, fatigue, and time pressure push the brain toward model-free responding, even when deliberation would help. This is why people under pressure fall back on habits, even bad ones.

The trade-off has a 500-million-year history. RL is rediscovering it.