Against Certainty
Think clearly when you don't know.
Against Certainty is a small field guide for thinking under uncertainty — with interactive mini-tools instead of theory. The premise: certainty is rarely on offer, but "less wrong" almost always is. Rather than hand you an answer, the tools show you how to reason with probabilities without needing the formulas. The Base-Rate Machine uses 100 people to show why a positive test is usually a false alarm — and that how rare the thing is tells you more about the result than how accurate the test is. The Panic-to-Probability Converter turns a concrete fear into a sober reading of how strong your evidence really is. "Your Risk in Micromorts" puts risks in proportion — and shows the dangerous thing is usually the boring one you do constantly. Plus essays that take apart one thinking error at a time, scenarios to play through, and fallacy cards for quick reference. English, free, no login. The tools run entirely in your browser; your inputs are never transmitted.
A positive rarely means what you think
One of the app's mini-tools, right here. Drag the sliders — the 100 tiles stand for 100 people and show why, for rare things, a positive test is usually a false alarm.
Before the test
4%
the base rate
After a positive
29%
chance it's real
Out of 100, 14 come back positive — but only 4 are actually the real thing.
A 90%-accurate test moved you from 4% to 29% — a real update, but a "positive" is still more likely a false alarm than the real thing. The rarer the thing, the more a single alarm lies.
Works for anything that flags rare things: medical screens, spam filters, fraud alerts, lie detectors. "Accurate" is one number standing in for both sides — how often the test is right when it is the real thing, and when it isn't.
- real & flagged
- false alarm
- real, missed
- correctly clear
Tech stack
- Next.js
- TypeScript
Platforms
- Web
Tags
- Rationality
- Decision Making
- Free
