For decades, students of advanced probability have faced a daunting rite of passage: cracking open David Williams’ (often abbreviated PwM). Published as part of the Cambridge Mathematical Textbooks series, this slim, unassuming volume is legendary—not just for its brilliant conciseness, but for its notoriously challenging exercises.

Iterating this argument, we conclude that $\mathbbE[X_n] = \mathbbE[X_1]$ for all $n \geq 1$.

David Williams had learned to read the world in probabilities. Growing up in a coastal town where fog rolled thicker than certainty, he found solace in numbers that measured chance—dice, coin flips, and later, conditional expectations that bent the future around present information. By his late twenties he was a young professor with a battered copy of a classic text on his desk and a quiet obsession: martingales.