Quantum mechanics is our best mathematical model of the universe to date, powerfully confirmed by a century of tests. However, interpreting what the experimental results mean—how and when the Schrödinger equation and Born’s rule interact—is a topic of much contention, with the main disagreement being between the Everett and the Copenhagen interpretations.

Yudkowsky uses this scientific controversy as a proving ground for some central ideas from previous sequences: map-territory distinctions, mysterious answers, Bayesianism, and Occam’s Razor.