On 26 March 2024, Daniel Kahneman sent an email to his closest friends. It was, by all accounts, a very Kahneman sort of email: precise, warm, entirely devoid of self-pity, and probably the most carefully considered goodbye letter in the history of modern psychology.

He told them he was on his way to Switzerland. He would not be coming back. He was ninety, his kidneys were failing, the mental lapses were increasing. “It is time to go,” he wrote. The next day, he died by assisted suicide in Zurich.

The week before, he had been in Paris with his family. They walked for hours, went to museums, ate soufflés and chocolate mousse. He wrote in the mornings. The evenings were for people he loved.

Philip Tetlock, a psychologist and old friend, said afterward: “I have never seen a better-planned death than the one Danny designed.”

If you have not heard of Kahneman, here is the short version. He was a psychologist who won the Nobel Prize in Economics in 2002, which is either a comment on the breadth of his thinking or on the narrowness of economics, possibly both.

His book Thinking, Fast and Slow, published in 2011, became one of those rare works that quietly rearranges how you see yourself. It sold millions. It deserved to. For me, he has been a north star. how we misread our own experience, how we are wrong in entirely predictable ways, has shaped how I think about almost everything. Losing him felt, in a way that is difficult to explain, like losing a teacher I never met.

The Man Who Understood How Memory Works

Kahneman spent his career proving that humans are bad at understanding their own experience. His most important finding was the distinction between the experiencing self, the one living through each moment, and the remembering self, the one who files the story away afterward. They are not the same person and they want different things.

Duration barely registers in memory. We judge an experience by its peak intensity and its ending. A holiday that ends with lost luggage is a bad holiday, regardless of the ten days of sunshine before it. A painful medical procedure that ends gently is remembered as less bad than a shorter one that ends badly. Kahneman called this the peak-end rule. He found it held not just for holidays but for entire lifetimes.

He had watched his mother decline into cognitive deterioration. He had watched his wife, Anne Treisman, die after years of vascular dementia. He had spent decades, in other words, studying the ending he did not want. So he did not wait for it. He applied his own research to his own life and, as one friend noted, “created a happy ending to a ninety-year life, in keeping with his peak-end rule.”

Meanwhile, in Silicon Valley

While Kahneman was eating soufflés in Paris, Bryan Johnson was monitoring his nighttime erections.

Johnson, 48, spends $2 million a year to reverse his biological age. He tracks over a hundred biomarkers, follows a strict vegan diet, sleeps at the same time every night, and has recently launched a programme called Immortals, offering three clients his exact anti-ageing protocol for $1 million a year each. His movement is called Don’t Die. He means it.

The longevity industry attracted nearly $8.5 billion in investment in 2024. Some of that money is funding genuinely interesting science. Some of it is funding Bryan Johnson, who spent a period receiving blood transfusions from his teenage son. If kids I know were his son, he would have some difficult questions.

One Species, Doing the Maths

Sometime ago, as strong black coffee woke me up, I read a mind-numbing statistic. One in five butterflies across the US has vanished since 2000. Insects vanish globally at one per cent every year.

From ten thousand kilometres away, the picture assembles itself without much effort. One species is spending billions to extend its tenure on earth indefinitely. Every other species is spending its last reserves simply trying to survive that one.

But I digress.

A Happy Ending

In his final letter, Kahneman wrote: “I discovered after making the decision that I am not afraid of not existing, and that I think of death as going to sleep and not waking up. So if you were inclined to be sorry for me, don’t be.”

He had been working on research papers the week he died. When asked what he would most like to do, near the end, he said he would like to learn something.

In the end, he did not ask for more time. He asked for a good ending. Soufflés in Paris, the childhood playground, the people he loved in the evenings. Then he left, on his own terms, while he still could.

Two years since he left, that was the clearest thinking he ever did.