What does it cost to follow an honest thought all the way to the end?

In 1280, a Jewish physician in Baghdad finished a book. He had examined Judaism, Christianity and Islam by a single standard, refusing to exempt his own faith, and somewhere near the end he had written one sentence that could destroy him. He looked at it by a lamp that was burning down. He could have struck it out. Nobody would ever have known there had been anything to strike.

He left it standing.

For four years, nothing happened. The book sat in a handful of houses, ignored. He went on seeing patients, buying paper, lighting the lamps on Friday evenings, growing old.

Then a preacher read that sentence aloud from a pulpit, and by that evening a crowd was outside his door calling for him to be burned.

His name was Ibn Kammuna. He was sixty-nine years old, and what happened next has almost never been told correctly.

Carried in Darkness is his recovered life - from the Jewish quarter of the greatest city on earth, through the Mongol catastrophe that destroyed it, to a leather-covered box on a night road and the four people whose names history did not bother to write down.

• Watch a physician's discipline become a philosophical method, and then a death sentence
• Understand the argument about the self that let a man lose everything and remain
• Discover the governor who faced a mob and saved him with a lie
• See why a book can sit harmless for four years and detonate in an afternoon

This is a book for anyone who has ever swallowed an honest thought because they feared the room.

The crowd is forgotten. Every one of them. The sentence is still here.

Open the first page.