Molaison was employed at a sheltered work center near Hartford. One of his tasks was packing balloons into small bags, stopping when the right number had been put in. But he could not remember what that number was. If asked to fetch a tool, he would forget which tool was needed before reaching the store; his supervisor began providing him with a picture of it. Personal hygiene is also memory-dependent, and one of Molaison’s caretakers took to leaving notes around the place to remind him to raise the toilet seat. Molaison could not remember when he had eaten, and the cues arising from his stomach were rarely sufficient to signal that he was either hungry or replete. He’d tell the same story to the same person over and over, unaware that he’d told it before. Molaison could not securely retain memories of the deaths of loved ones, so each confrontation with the fact of a long-ago passing was as raw as the first. Unable to keep in mind what had happened to his parents, he put notes in his wallet telling him that his father was dead and that his mother was in a nursing home. But, without the notes, he found it hard to remember where his parents were or if they were dead or alive.

When you asked Molaison a question, he could retain it long enough to answer; when he was eating French toast, he could remember previous mouthfuls and could see the evidence that he had started eating it. His unimpaired ability to do these sort of things illustrated a distinction, made by William James, between “primary” and “secondary” memory. Primary memory, now generally known as working memory, evidently did not depend upon the structures that Scoville had removed. The domain of working memory is a hybrid of the instantaneous present and of what James referred to as the “just past.” The experienced present has duration; it is not a point but a plateau. For those few seconds of the precisely now and the just past, the present is unarchived, accessible without conscious search. Beyond that, we have to call up the fragments of past presents. The plateau of Molaison’s working memory was between thirty and sixty seconds long—not very different from that of most people—and this was what allowed him to eat a meal, read the newspaper, solve endless crossword puzzles, and carry on a conversation. But nothing that happened on the plateau of working memory stuck, and his past presents laid down no sediments that could be dredged up by any future presents.

There was a sense in which Molaison was able to learn, though it’s probably wrong to say that he learned things. In a way that’s not adequately understood, but which may have to do with the fact that some semantic learning is sustained by structures that Scoville left intact, he seemed occasionally to acquire odd pieces of new factual knowledge—for example, that there were things called contact lenses and that there was a famous person named Yoko Ono, though he was not too clear what kind of person that was (“an important man in Japan”). As he aged, he learned to use a walker; as time went on, he gradually learned how to move around new environments and even to acquire what seems to have been a mental map of new places that had, in some way, become familiar. He might not have been able to supply an answer to a question about just where he was, but he acted, so to speak, as if he knew, and could take account of, its corners and sharp edges.

In 1945, the philosopher Gilbert Ryle distinguished between “knowing how” (procedural knowledge of the sort that’s involved in riding a bike) and “knowing that” (for example, the ability to acquire, archive, and retrieve facts about how a bike works). In one striking experiment, Molaison was asked repeatedly to trace a star in an apparatus that insured that he could see the figure and his hand only in a mirror. Each time he took the test, he thought that he had never done it before, and yet his accuracy markedly improved through iterations. “Well, this is strange,” he said. “I thought that that would be difficult. But it seems as though I’ve done it quite well.” In the language of modern psychology, his improvement in the star-tracing experiment belonged to “non-declarative” memory: Molaison retained an ability to learn motor skills, so scientists concluded that these capacities resided not in the hippocampus and nearby structures but elsewhere in the brain. Molaison’s inadvertent gift to science was helping researchers understand that memory is not a single process, and that its various capacities do not reside in a single cerebral structure.

Neuroscientists describe Molaison as “arguably the single most important patient ever studied in neuropsychology”; his was “the brain that changed everything,” “the most famous brain in the world.” Major careers in neuroscience were built on Molaison’s condition and on access to him as an experimental subject, and no career was more substantially defined by Molaison than that of Suzanne Corkin.

Corkin writes that Henry Molaison was “part of my life for decades.” Forty-six years, to be exact. In 1961, as a graduate student at McGill University, she joined the laboratory of a colleague of Scoville’s, and met Molaison the following year. After moving to M.I.T. a few years later and setting up her own neuroscience lab, Corkin began a scientific and legal association with Molaison that’s unique in the history of science. The Clinical Research Center at M.I.T. became, Corkin writes, Molaison’s “home away from home,” and the scientists interested in investigating his brain and his behavior became his “family.” Between 1966 and 2000, Molaison visited Corkin’s lab fifty-five times for testing and observation, sometimes staying at M.I.T. for weeks at a time. These visits became, another writer noted, “the only salient feature of his life.” Molaison wound up becoming Corkin’s legal responsibility. When he was admitted to a long-term-care facility, Molaison’s hospital chart listed Corkin as “the only interested relative, friend, or contact.”

By the late seventies, Corkin legally controlled access to the patient. There was no end of investigators dying for a chance to meet Molaison, but Corkin required any scientist wishing to study Molaison “to visit my lab first and present the proposed research protocol at our weekly lab meeting.” In the course of fifty years, about a hundred scientists got to experiment on Molaison; many others were turned away. With great difficulty, the science journalist Philip Hilts persuaded Corkin to grant him access, and his moving book “Memory’s Ghost” (1995) was the first account of Molaison that was written for a general audience. The staff at Molaison’s care facility was directed to keep quiet about him, neither confirming nor denying the presence of any such person. Toward the end of his life, Molaison’s problems had come to include dementia, and Corkin made arrangements for a postmortem brain donation, which she regards as “a beautiful finale to his enduring contributions” to science. Corkin drafted a set of rules governing the handling of Molaison’s death. They were attached to his medical chart, and stipulated that she must be contacted immediately as death approached.

“Permanent Present Tense” is about Molaison’s mind as a resource for understanding how human minds normally work and how memory is normally processed and stored. “Studying how Henry forgot gave us a better understanding of how we remember,” Corkin writes. She isn’t at ease with existential questions, but there’s a chapter called “Henry’s Emotional Life,” which offers her view that he did indeed have one. He could not track his own aging in the way that is, sadly, normal for the rest of us. A psychiatrist who was asked to examine Molaison reported that “he did not think of death.” Molaison could recognize his aging face in the mirror, but he could not associate that changing image with a history of his own body and mind. Once, he was looking in the mirror when a nurse asked him, “What do you think about how you look?” He replied, “I’m not a boy.” When Molaison was more than fifty years old, Corkin inquired how old he thought he was, and he responded, “Around thirty.” She then got out a mirror and asked him again; this time the answer was “Maybe forty.” Molaison’s aging body necessarily reflected the passage of time, while the mirror image that he saw, playing the role of the painting of Dorian Gray, did not.