Most decisions in your life are judged by their outcome. They shouldn't be.

Blackjack is the tractable case. The rules are closed, the probabilities are public, and the outcome arrives in seconds. Every hand is a self-contained experiment: a decision, a distribution, a realised result, and enough structure to tell them apart.

The maths behind each hand is not the point. The habit is. Once you can look at a hand you lost and say “the decision was still the strongest one available” — and mean it — you have something portable.

Two things, kept separate on purpose:

• Decision quality. How your choice compares to the strongest action from the same information state. Judged against the distribution, never against the card that fell.
• Outcome. What actually happened. Recorded honestly, never blended into the decision score.

The two are shown side by side after every hand. They are never combined into a single number.

• Not a casino. No stakes, no money, no chips.
• Not a card-counting drill. The engine reads only what any player at the table can see.
• Not a lecture. The demo below runs in seconds; the rest is opt-in.

The frame carries. A trial that failed on a reasonable protocol. A hire that didn't work out for reasons the interview couldn't have surfaced. A trade sized correctly that still lost. In each, the temptation is the same: rewrite the earlier decision using information that arrived later.

Sextillion does not claim to teach medicine, hiring, or markets. It offers one small, honest practice ground where the distinction is legible, and lets you keep the reflex.