Big ideas you actually use.

CODEX is an Android app that teaches mental models, cognitive biases, stoicism, persuasion and strategy as a journey. Each idea is a short card you read in about four minutes, followed by a three-question quiz and a field mission that asks you to try the idea in real life.

Mental models are compact explanations of how something works, such as the 80/20 principle, inversion or loss aversion. Having many of them lets you look at a problem from several angles and avoid predictable mistakes. CODEX collects them from thinkers like Charlie Munger, Daniel Kahneman, Sun Tzu, Marcus Aurelius and Nassim Taleb.

Yes, CODEX is free to download. Five full decks are free (Sun Tzu, Taleb, Greene, Cipolla and Patterns in Yourself), and the first card of every other deck is free to read. CODEX PRO, a subscription, unlocks all 21 decks and unlimited reflections.

The yearly PRO plan starts with a 7-day free trial for new subscribers. You can cancel at any time in Google Play before the trial ends and you will not be charged. A weekly plan is also available, without a trial.

Every card has quiz questions with explanations, and a spaced review brings back the ideas you are about to forget. The Daily Challenge gives everyone the same 10 questions each day. Weekly leagues go from Bronze to Diamond: finish near the top of your group to move up. Scores are computed on the server so the boards stay fair.

No. You can read, take quizzes and review as a guest, and most of the app works offline. Signing in with email or Google is optional and lets you sync progress across devices and keep your subscription when you change phones.

The cards and quizzes are in English for now.

In the app, go to You, then Delete my data. If you had an account, you can also request deletion from our account deletion page, linked at the bottom of every page.