Ronald Dworkin, an influential legal scholar and the author of “Life’s Dominion: An Argument About Abortion, Euthanasia and Individual Freedom,” wrote about a kind of hierarchy of needs for people in Sandy’s situation, who want their autonomy to be respected even as disease changes the essence of who they are and what autonomy means. He differentiated between “critical interests” (personal goals and desires that make life worth living) and “experiential interests” (enjoying listening to music, for instance, or eating chocolate ice cream). Sandy was appreciating her experiential interests — playing with Felix and working in her garden — but her critical interests were far more sophisticated and were moving out of her reach. Critical interests should take priority when making end-­of-­life choices on behalf of someone whose changed state renders her less capable of deciding on her own, Dworkin wrote, because critical interests reflect your true identity. The new Sandy seemed to love being a grandmother, but it was important to take into account what the old Sandy would have wanted.

Granting priority to critical interests is difficult even in a society that tries to do so. In the Netherlands, the Termination of Life on Request and Assisted Suicide Act makes it possible for a doctor to end a person’s life when she is not cognitively able to do it herself, as long as she laid out her intention while she was still competent. According to the 2002 law, if someone with Alzheimer’s disease has an advance directive declaring her wish to die when her dementia reaches a point she considers intolerable — when she has to be spoon-­fed, for example, or put in diapers — that document is sufficient to allow a doctor to perform euthanasia. Nevertheless, it is rare for a doctor in the Netherlands to actually euthanize a patient who has dementia. In fact, one recent survey of 110 Dutch physicians treating dementia patients with advance directives asking to be euthanized found not a single one who had carried out the request. And of the 4,829 people who died in 2013 under the Dutch euthanasia act, just 97 of them, or 2 percent, had dementia.

“You know I plan to kill myself,” Sandy said all through 2013, whenever the thought occurred to her. She seemed to say it partly for the sake of others, so they could get used to the idea and steel themselves against pain and grief when the time came. But it seemed that it was also for her own sake, to keep her plan at the forefront of her disintegrating mind. Emily and Felix were living with Sandy at the time, so that Sandy could help with child care while Julius attended nursing school in Colorado. (Jeremy had recently moved out. He was going through a rocky time and was not communicating with the family, though he supported his mother’s plans.) It drove Emily crazy to hear her mother continually bring up suicide. “Stop saying that!” she would tell her.

One night in August 2013, when Sandy was home alone, she pulled out a yellow legal pad and sat down at the tile-­topped table in her big, oak-­trimmed kitchen, where she had eaten thousands of dinners. She had just heard of two experimental treatments for Alzheimer’s that she hoped might, in addition to the Namenda and Aricept, keep her functioning so that she could help care for Felix until August 2014, when Julius was scheduled to finish his nursing program and move back home. But the drugs were prohibitively expensive, and she would have to pay for them out of pocket, because her insurance wouldn’t cover the cost. On the pad, she started to make calculations. The treatment — a combination of IVIG (intravenous immunoglobulin), a drug approved for other neurodegenerative diseases, and repetitive TMS (transcranial magnetic stimulation), which was usually prescribed for depression — cost $6,000 every two weeks at the New York Memory and Healthy Aging Services. What if she could persuade the center to charge less, because at her size she would need less medication? And what if she received the treatment less frequently, maybe every three weeks? It was still a lot of money, but she had never touched her I.R.A., and she was already 69 and was clearly not going to live much longer. She calculated that her savings could cover about $4,000 every three weeks until Julius graduated in a year. At the bottom of the page filled with numbers, she wrote to remind herself not to fret too much over the staggering dollar amount. “Expensive: but now money is not an issue (because of imily).” The previously meticulous scholar had misspelled her daughter’s name.

Over the next months, Sandy and Daryl boarded a bus early in the morning every few weeks and rode down to Manhattan for the treatments. “I still feel as though I’m me,” she told him on one ride. “Do you agree?” He did, sort of. In fact, he was surprised by how much herself Sandy could still be, even as she became less and less the formidable thinker he had always known. He was surprised too to discover that it didn’t matter to him. “I realized how little of the fact that she was an intellectual played into my feelings for her,” he said. “They were feelings for her, not her intelligence. And they were still all there.”

Daryl proved himself steadfast, and as her more casual friends fell away, either because Sandy shut them out or because they were unwilling to witness her decline, he became more central to her life. He and Karen were the ones who saw her frequently, and they were the ones she kept checking in with to be sure her suicide window was not about to close.

In October, Sandy wrote to an address in Mexico listed on the website of “The Peaceful Pill Handbook.” Weeks passed, and she fretted that her order had been confiscated at the border. But at last it arrived: a cardboard box, no bigger than a softball, wrapped in brown paper. Sandy eagerly took scissors to the packaging and retrieved two 100-­milliliter bottles of pentobarbital — she had bought an extra one just in case, even though she believed that one bottle would be enough for a person her size. The drug needed to be kept in a cool place, so she took the bottles down to the basement. For the time being, she could leave the pentobarbital on a shelf, comforted by the knowledge that it was there.