Frost pushed the play button on the phone screen. He expected a dry academic lecture in a classroom, but instead, he saw a video of an urban street somewhere in San Francisco. There were cars parked on the opposite curb. The street was lined with retail shops. Pedestrians walked back and forth in groups on both sides. As he watched, puzzled, a dark car drove into the frame and went without stopping through the intersection, where it T-boned another car with a sharp bang. Steam erupted. Voices shouted. And then the video cut off.

“I don’t understand,” Frost said.

“Let’s say you witnessed this actual incident,” Stein said, taking back her phone. “That ten seconds would be your reality. You can’t reexperience it, you can’t watch it again. All you can do is remember it.”

“Okay.”

“In other words, reality happens once, but memory happens over and over,” Stein told him. “Every time I ask you to think about the blue car that zipped through the stop sign and had an accident, your brain goes back and retrieves the memory, like a file from a cabinet. However, memories—unlike reality—aren’t fixed. With every recollection, we reshape what we saw. Our memories of an event are influenced by how we want a situation to be, how we perceive our role in it, what people tell us, and even by what we hear or read about what took place. After a while, our brains can’t distinguish between reality and our reconstruction of reality.”

“Eyewitnesses are unreliable,” Frost said. “I get it.”

“Exactly. Not only are they unreliable, they can be stubborn about it, too. Witnesses are often one-hundred-percent convinced of the facts, even when they’re wrong. And trauma can actually make it worse. You wouldn’t think a rape victim could ever misidentify her assailant, right? And yet it happens. Innocent men have gone to prison because of it.”

“Like I said, people get it wrong. How does that relate to what you do?”

Stein responded with a slight dip of her chin. She had a calmness and precision in everything she did. “My point is that people can change their own memories without even being aware that they’re doing so. The danger—and the opportunity—is that memories can also be deliberately altered. You may have heard about a controversy back in the nineteen eighties, in which therapists helped patients recover repressed memories of abuse. Most of those recovered memories were discredited, but to the patient they became real. And it’s not just therapists who are guilty of this kind of manipulation. Attorneys do the same thing, and so do police officers. Sometimes it’s accidental, and sometimes it’s intentional.”

“How does that work?” Frost asked.