In mid-March 2024, Daniel Kahneman flew from New York to Paris to unite with his wife and daughter. They spent days walking around the city, going to museums and the ballet, and savoring soufflés and chocolate mousse. Around March 22, Kahneman, who had turned 90 that month, also started emailing a personal message to several dozen of the people he was closest to.

On March 26, Kahneman left his family and flew to Switzerland. His email explained why: This is a goodbye letter I am sending friends to tell them that I am on my way to Switzerland, where my life will end on March 27.

Kahneman was one of the world’s most influential thinkers—a psychologist at Princeton University, winner of the Nobel Prize in economics and author of the international blockbuster “Thinking, Fast and Slow,” first published in 2011. He had spent his long career studying the imperfections and inconsistencies of human decision-making. By most accounts—although not his own—Kahneman was still in reasonably good physical and mental health when he chose to die.

Why did he end his life? One friend said, “I think Danny wanted, above all, to avoid a long decline, to go out on his terms, to own his own death. Maybe the principles of good decision-making that he had so long espoused—rely on data, don’t trust most intuitions, view the evidence in the broadest possible perspective—had little to do with his decision.

Another friend said, “My best guess is he felt he was falling apart, cognitively and physically. And he really wanted to enjoy life and expected life to become decreasingly enjoyable. I suspect he worked out a hedonic calculus of when the burdens of life would begin to outweigh the benefits—and he probably foresaw a very steep decline in his early 90s.”