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Without a memory, EP has fallen completely out of time. He has no stream of consciousness, just droplets that immediately evaporate. If you were to take the watch off his wrist—or, more cruelly, change the time—he'd be completely lost. Trapped in this limbo of an eternal present, between a past he can't remember and a future he can't contemplate, he lives a sedentary life, completely free from worry. "He's happy all the time. Very happy. I guess it's because he doesn't have any stress in his life," says his daughter Carol, who lives nearby.

"How old are you now?" Squire asks him.

"Let's see, 59 or 60. You got me. My memory is not that perfect. It's pretty good, but sometimes people ask me questions that I just don't get. I'm sure you have that sometimes."

"Sure, I do," says Squire, kindly, even though EP is almost a quarter of a century off.

An enormous amount of what science knows about memory was learned from a damaged brain that is remarkably similar to EP's. It belongs to an 81-year-old man known as "HM," an amnesiac who lives in a nursing home in Connecticut. As a child, HM suffered from epilepsy that began after a bike accident at age nine. By the time he was 27, he was blacking out ten times a week and unable to do much of anything. A neurosurgeon named William Scoville thought he could cure HM's epilepsy with an experimental surgery that would excise the part of the brain that he suspected was causing the problem.

In 1953, while HM lay awake on the operating table, his scalp anesthetized, Scoville drilled a pair of holes just above the patient's eyes. The surgeon lifted the front of HM's brain with a small metal spatula while a metal straw sucked out most of the hippocampus, along with much of the surrounding medial temporal lobes. The surgery reduced the number of HM's seizures, but it soon became clear that he'd also been robbed of his memory.

Over the next five decades, HM was the subject of countless experiments and became the most studied patient in the history of brain science. Given the horrific outcome of Scoville's surgery, everyone assumed HM would be a singular case study.

EP shattered that assumption. What Scoville did to HM with a metal straw, nature did to EP with herpes simplex. Side by side, the grainy black-and-white MRIs of their brains are uncannily similar, though EP's damage is a bit more extensive. Even if you have no idea what a normal brain ought to look like, the gaping symmetrical holes stare back at you like eyes.

Like EP, HM was able to hold on to memories just long enough to think about them, but once his brain moved to something else, he could never bring them back. In one famous experiment, Brenda Milner, a Canadian psychologist, asked HM to remember the number 584 for as long as possible. To keep the number on the tip of his tongue, he used a complicated system, which he recounted to Milner:

"It's easy. You just remember 8. You see 5, 8, and 4 add to 17. You remember 8, subtract it from 17, and it leaves 9. Divide 9 in half and you get 5 and 4 and there you are: 584. Easy."

He concentrated on this elaborate mantra for several minutes. But as soon as he was distracted, the number dissolved. He couldn't even remember that he'd been asked to remember something. Though scientists had known that there was a difference between long- and short-term memory since the late 19th century, they now had evidence in HM that the two types of memory happened in different parts of the brain, and that without most of the hippocampal area, HM couldn't turn a short-term memory into a long-term one.

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