The 50% rule

If you're at 50% certainty, you don't know. If you're at 80%, you might. If you're at 95%, you're probably wrong about at least one hidden assumption.

Calibration isn't a natural human skill. Most people are over-confident at the high end and under-confident at the low end. Practicing deliberate calibration changes how you make decisions.

The scale

At 50% certainty, you're flipping coins. You don't know. If you're pretending you do, you're fooling yourself.

At 80%, you have a reasonable view. You should act on it but keep your eye out for new information.

At 95%, you're confident enough that you should be alarmed by one thing: hidden assumptions. Very high confidence usually means you haven't thought about what could make you wrong. When you actually list the assumptions, many of them are less certain than your overall take.

At 99%, you're either genuinely right about something simple, or you're wrong and don't realize it.

The exercise

For any meaningful decision, force yourself to assign a probability. Write it down. Then, separately, list what would have to be true for the opposite to be the case.

If you confidently assigned 95%, and the opposite list has even one plausible item you hadn't deeply examined, your 95% was wrong. You meant 80%.

Over time, this practice recalibrates your gut.