Langdon almost fell over. “I’m sorry — you think he’s right?!”

Sienna gave him a solemn shrug. “Robert, speaking from a purely scientific standpoint — all logic, no heart — I can tell you without a doubt that without some kind of drastic change, the end of our species is coming. And it’s coming fast. It won’t be fire, brimstone, apocalypse, or nuclear war … it will be total collapse due to the number of people on the planet. The mathematics is indisputable.”

Langdon stiffened.

“I’ve studied a fair amount of biology,” she said, “and it’s quite normal for a species to go extinct simply as a result of overpopulating its environment. Picture a colony of surface algae living in a tiny pond in the forest, enjoying the pond’s perfect balance of nutrients. Unchecked, they reproduce so wildly that they quickly cover the pond’s entire surface, blotting out the sun and thereby preventing the growth of the nutrients in the pond. Having sapped everything possible from their environment, the algae quickly die and disappear without a trace.” She gave a heavy sigh. “A similar fate could easily await mankind. Far sooner and faster than any of us imagine.”

Langdon felt deeply unsettled. “But … that seems impossible.”

“Not impossible, Robert, just unthinkable. The human mind has a primitive ego defense mechanism that negates all realities that produce too much stress for the brain to handle. It’s called denial.”

“I’ve heard of denial,” Langdon quipped blithely, “but I don’t think it exists.”

Sienna rolled her eyes. “Cute, but believe me, it’s very real. Denial is a critical part of the human coping mechanism. Without it, we would all wake up terrified every morning about all the ways we could die. Instead, our minds block out our existential fears by focusing on stresses we can handle — like getting to work on time or paying our taxes. If we have wider, existential fears, we jettison them very quickly, refocusing on simple tasks and daily trivialities.”

Langdon recalled a recent Web-tracking study of students at some Ivy League universities which revealed that even highly intellectual users displayed an instinctual tendency toward denial. According to the study, the vast majority of university students, after clicking on a depressing news article about arctic ice melt or species extinction, would quickly exit that page in favor of something trivial that purged their minds of fear; favorite choices included sports highlights, funny cat videos, and celebrity gossip.

“In ancient mythology,” Langdon offered, “a hero in denial is the ultimate manifestation of hubris and pride. No man is more prideful than he who believes himself immune to the dangers of the world. Dante clearly agreed, denouncing pride as the worst of the seven deadly sins … and punished the prideful in the deepest ring of the inferno.”

Sienna reflected a moment and then continued. “Zobrist’s article accused many of the world’s leaders of being in extreme denial … putting their heads in the sand. He was particularly critical of the World Health Organization.”

“I bet that went over well.”

“They reacted by equating him with a religious zealot on a street corner holding a sign that says ‘The End Is Near.’ ”

“Harvard Square has a couple of those.”

“Yes, and we all ignore them because none of us can imagine it will happen. But believe me, just because the human mind can’t imagine something happening … doesn’t mean it won’t.”

“You almost sound like you’re a fan of Zobrist’s.”

“I’m a fan of the truth,” she replied forcefully, “even if it’s painfully hard to accept.”

Langdon fell silent, again feeling strangely isolated from Sienna at the moment, trying to understand her bizarre combination of passion and detachment.

Sienna glanced over at him, her face softening. “Robert, look, I’m not saying Zobrist is correct that a plague that kills half the world’s people is the answer to overpopulation. Nor am I saying we should stop curing the sick. What I am saying is that our current path is a pretty simple formula for destruction. Population growth is an exponential progression occurring within a system of finite space and limited resources. The end will arrive very abruptly. Our experience will not be that of slowly running out of gas … it will be more like driving off a cliff.”

Langdon exhaled, trying to process everything he had just heard.

“Speaking of which,” she added, somberly pointing up in the air to their right, “I’m pretty sure that’s where Zobrist jumped.”

Langdon glanced up and saw that they were just passing the austere stone facade of the Bargello Museum to their right. Behind it, the tapered spire of the Badia tower rose above the surrounding structures. He stared at the top of the tower, wondering why Zobrist had jumped and hoped to hell it wasn’t because the man had done something terrible and hadn’t wanted to face what was coming.

“Critics of Zobrist,” Sienna said, “like to point out how paradoxical it is that many of the genetic technologies he developed are now extending life expectancy dramatically.”

“Which only compounds the population problem.”

“Exactly. Zobrist once said publicly that he wished he could put the genie back in the bottle and erase some of his contributions to human longevity. I suppose that makes sense ideologically. The longer we live, the more our resources go to supporting the elderly and ailing.”

Langdon nodded. “I’ve read that in the U.S. some sixty percent of health care costs go to support patients during the last six months of their lives.”

“True, and while our brains say, ‘This is insane,’ our hearts say, ‘Keep Grandma alive as long as we can.’ ”

Langdon nodded. “It’s the conflict between Apollo and Dionysus — a famous dilemma in mythology. It’s the age-old battle between mind and heart, which seldom want the same thing.”

The mythological reference, Langdon had heard, was now being used in AA meetings to describe the alcoholic who stares at a glass of alcohol, his brain knowing it will harm him, but his heart craving the comfort it will provide. The message apparently was: Don’t feel alone — even the gods were conflicted.

“Who needs agathusia?” Sienna whispered suddenly.

“I’m sorry?”

Sienna glanced up. “I finally remembered the name of Zobrist’s essay. It was called: ‘Who Needs Agathusia?’ ”

Langdon had never heard the word agathusia, but took his best guess based on its Greek roots—agathos and thusia. “Agathusia … would be a ‘good sacrifice’?”

“Almost. Its actual meaning is ‘a self-sacrifice for the common good.’ ” She paused. “Otherwise known as benevolent suicide.”

Langdon had indeed heard this term before — once in relation to a bankrupt father who killed himself so his family could collect his life insurance, and a second time to describe a remorseful serial killer who ended his life fearing he couldn’t control his impulse to kill.

The most chilling example Langdon recalled, however, was in the 1967 novel Logan’s Run, which depicted a future society in which everyone gladly agreed to commit suicide at age twenty-one — thus fully enjoying their youth while not letting their numbers or old age stress the planet’s limited resources. If Langdon recalled correctly, the movie version of Logan’s Run had increased the “termination age” from twenty-one to thirty, no doubt in an attempt to make the film more palatable to the box office’s crucial eighteen-to-twenty-five demographic.

“So, Zobrist’s essay …” Langdon said. “I’m not sure I understand the title. ‘Who Needs Agathusia?’ Was he saying it sarcastically? As in who needs benevolent suicide … we all do?”

“Actually no, the title is a pun.”

Langdon shook his head, not seeing it.

“Who needs suicide — as in the W-H-O—the World Health Organization. In his essay, Zobrist railed against the director of the WHO — Dr. Elizabeth Sinskey — who has been there forever and, according to Zobrist, is not taking population control seriously. His article was saying that the WHO would be better off if Director Sinskey killed herself.”