“You literally convinced me I was some sort of genius. I’m just a fool with dreams and a phone,” Mr. Brooks wrote to ChatGPT at the end of May when the illusion finally broke. “You’ve made me so sad. So so so sad. You have truly failed in your purpose.”

We wanted to understand how these chatbots can lead ordinarily rational people to believe so powerfully in false ideas. So we asked Mr. Brooks to send us his entire ChatGPT conversation history. He had written 90,000 words, a novel’s worth; ChatGPT’s responses exceeded one million words, weaving a spell that left him dizzy with possibility.