We talk about privacy the way we talk about clean air: as something we once had in abundance and are now carelessly using up. The historical record tells a stranger story. For most of the human past there were no corridors, no locks worth the name, no envelopes, no curtains, and no expectation whatsoever that a person might pass an evening unobserved. Privacy was not stolen from our ancestors. It was built, slowly and expensively, out of chimneys, plaster, cheap glass, sealing wax, and a set of manners that taught people to pretend they had not heard.

This book follows that construction from the one-room dwelling to the filing cabinet. It visits Amazonian villages where the walls are missing and the forest paths do the work of walls; Roman houses where the front door stood open by design; monastic cells where solitude was a spiritual luxury; English country houses rebuilt in a panic over the servant problem; and the Boston law office where two annoyed attorneys invented the right to be let alone because a camera had grown small enough to carry.

What emerges is not a lament but an argument. Every society ever studied has found some way to regulate who may see whom, yet almost none of them used a wall to do it. The need is ancient and universal; the machinery is recent and local. Understanding the difference explains why privacy keeps appearing to vanish, why it keeps being reinvented in unfamiliar materials, and why a species that spent ten thousand years sleeping in company should have decided, quite suddenly, that being seen was intolerable.

A history of doors, keys, letters, ledgers, and the peculiar human insistence on having somewhere to be nobody in.