An interactive tutorial · six modules
The arithmetic of changing your mind
Bayesian thinking, taught by instruments you operate. Move a slider, count the outcomes, watch your belief update — then read why it moved.
The course of the argument
Beliefs as bets
Put numbers on what you believe — probability as degree of confidence, and odds as its native dialect.
Module IIBayes by counting
Solve the famous medical-test puzzle by counting people in a grid. No formula — arithmetic you already know.
Module IIIPriors & base rates
Why where you start matters, why ignoring it is the classic blunder, and why a prior is not a bias.
Module IVWeighing evidence
How strong is a clue? Likelihood ratios, odds-form updating — and the notation, finally earned.
Module VUpdating in sequence
Evidence arrives one draw at a time. Watch a whole curve of belief sharpen as the data accumulates.
Module VIBayes in the wild
Spam filters, A/B tests, forecasting, everyday judgment — the same arithmetic, out of the laboratory.
Every concept arrives as a panel you operate first and read about second. The formula itself doesn’t appear until Module IV — by then you’ll have computed it three times without noticing.