I had heard of DID before. It’s the staple of many a horror plot. But watching Encina’s system describe how their personality had diverged in order to deal with abuse, I felt that a piece of the world I had never understood had fallen into place. The world felt stranger now, but also more real. It was a kind of miracle, but it also made perfect sense, that the mind should do this.

I rang a friend of mine who is a psychotherapist. She has worked with, among others, survivors of trafficking and torture. “Is this real?” I asked. “I mean, is this a real thing?” I wasn’t very articulate.

“In my experience it’s absolutely real,” she said.

For over a year I went down a long rabbit hole, reading everything I could get my hands on to do with DID. I suddenly understood what the book was, and where it needed to go.

There are people in the therapeutic community and the world at large who firmly believe the disorder doesn’t exist. DID seems to threaten people’s worldview. Maybe it’s because it interferes with the concept of the soul—the idea that there can be more than one person in a body is somehow terrifying. It certainly disrupts the underlying tenets of many religions.

The stories that accompany this disorder are without exception horrific. It’s the mind’s last resort, when faced with unbearable pain and fear. I am particularly grateful to First Person Plural, one of the major support groups for people with dissociative identity disorder in the UK, for helping me better understand this intricate condition. Their website and online resources are listed at the back of the book.

I spoke with someone who has dissociative identity disorder and works with others who have it, over the course of a long afternoon. They have asked not to be named. We met for the first time at a train station and went to a café nearby to talk. We were both flustered and shy at first. It’s an intimate thing to discuss between strangers. But they laid open their past, and their life, with unflinching honesty.