When Kohei Saito decided to write about “degrowth communism,” his editor was understandably skeptical. Communism is unpopular in Japan. Economic growth is gospel.

So a book arguing that Japan should view its current condition of population decline and economic stagnation not as a crisis, but as an opportunity for Marxist reinvention, sounded like a tough sell.

But sell it has. Since its release in 2020, Mr. Saito’s book “Capital in the Anthropocene” has sold more than 500,000 copies, exceeding his wildest imaginings. Mr. Saito, a philosophy professor at the University of Tokyo, appears regularly in Japanese media to discuss his ideas. His book has been translated into several languages, with an English edition to be issued early next year.

Mr. Saito has tapped into what he describes as a growing disillusionment in Japan with capitalism’s ability to solve the problems people see around them, whether caring for the country’s growing older population, stemming rising inequality or mitigating climate change.

Japan, the world’s third-largest economy, has worked for years to promote economic growth in the shadow of an aging, shrinking population, with a monetary and fiscal policy that is among the most aggressive of any nation.

But there are strong indications that the country’s growth-oriented policies of ultracheap money and big government spending are reaching their limits. The interventions have done little to stimulate growth in Japan’s economy. And as government efforts to lift the birthrate also falter, with fewer people doing less work, “the room for growth is running out,” Mr. Saito, 36, said during a recent interview at his Tokyo home.

That’s seemingly true even when Japan’s economy expands. When the country reported growth of 6 percent in the second quarter of this year, it was driven almost entirely by external factors: exports and inbound tourism. Domestic consumption, on the other hand, shrank.

The focus on growth was important when Japan was developing. But now that the country is wealthy, Mr. Saito said, the insistence on an endlessly expanding economy, described in terms of gross domestic product, or G.D.P., has produced obviously wasteful spending as the government has urged people to consume more.

Some areas of the economy, such as health care, will need to continue growing, but “there are too many cars, too many skyscrapers, too many convenience stores, too much fast fashion,” he said. The focus on consumption, he argues, has had devastating consequences for the environment, driven widening inequality and wasted limited resources that could be put to better use.

Reorienting Japan toward goals that more effectively reflect the country’s current needs, he says, would mean using metrics other than G.D.P. to gauge the country’s economic well-being. The focus would shift from quantity to quality, on measures like health, education and standard of living.

Mr. Saito first encountered Marx in 2005, when he was an undergraduate at the University of Tokyo. In high school, Mr. Saito was “more right wing,” he said, convinced that individual failings were the root cause of Japan’s problems. When he encountered the German philosopher’s arguments that structural causes led to inequality and war, it was “shocking,” he said.

“After the economic crisis of 2008, there was a Marx renaissance in Japan, and I was convinced of the importance of his theory,” Mr. Saito said.

He has spent the years since studying Marx’s twilight years, when, Mr. Saito argues, the philosopher realized that capitalism, with its insatiable demand for growth, would inevitably lead to environmental disaster.

Mr. Saito conceived “Capital in the Anthropocene” — a reference to an era in which human activity has a profound impact on the Earth’s environment — early in the Covid pandemic. Socialism was a hot topic in Europe and the United States, where politicians like Bernie Sanders urged Americans to grapple with the drawbacks of U.S.-style capitalism. The aftermath of the 2008 financial crisis, rising inequality and the unavoidable realities of climate change were driving many young people to question the sustainability and fairness of existing economic systems.

People in Japan, too, felt dissatisfied with the status quo, Mr. Saito said. But unlike people in other parts of the world, “they aren’t thinking, ‘Capitalism is bad,’ they’re thinking, ‘I’m bad.’ They aren’t thinking that capitalism needs to change, they’re thinking, ‘I need to change.’”

He recognized the thinking as similar to his own in high school, when he believed that people merely needed to work harder or be more productive.

Mr. Saito’s critics have called him out for castigating the capitalist system he himself has benefited from while providing little more than unworkable idealism and failed ideology as an alternative. His book has ignited a publishing boomlet on Marxism in Japan, with some works attacking his ideas and others supporting them.

The renewed discussion hasn’t done much to revive the prospects of Japan’s own Communist Party, however. Mr. Saito is not a fan of the group, which he sees as well-meaning but stale. He also does not have much patience for other more familiar strains of communism, such as that practiced by the Soviet Union and the Chinese Communist Party, with their emphasis on state power over industry and centralized planning.

He recognizes that growth remains crucial for improving quality of life in less developed countries. And even in rich nations, he does not call for people to give up their creature comforts. He recently moved into a three-story home in an upscale neighborhood on the outskirts of Tokyo and drives a compact Toyota. One of the few things he has given up, he said, is fast food.

Achieving degrowth communism, he believes, is less about personal choices and more about changing overarching political and economic structures. Marxism, he argues, offers a viable model for reorienting society around the maximization of public goods as opposed to the endless pursuit and concentration of wealth.

That would require, among other things, moving away from G.D.P. as the key measure of a country’s health. As an alternative, he suggests the “human development index,” an idea proposed by the Pakistani economist Mahbub ul Haq, which the United Nations has used as an alternative indicator of a country’s progress.

The index — which measures life expectancy, education and quality of life — gives a more comprehensive view of how the economy affects people’s lives than G.D.P.

Mr. Saito is not clear exactly what shape a world under degrowth communism would take, but he insists that it would be democratic and focused on expanding communal resources, reducing the wealth gap and removing incentives for excess consumption.

For his own part, he is participating in a handful of projects aimed at promoting those ideas. He and a group of supporters are purchasing land in the mountains west of Tokyo, which they plan to run as a collective to benefit the local community.

And for the last year, he has spent time on an organic farm outside Tokyo that has positioned itself less as a business and more as a community resource for urbanites to get healthy food and learn about agriculture.

The farm itself is, in one sense, a glimpse of a post-growth Japan where a shrinking population finds itself left with an abundance of resources. The fields are pieced together from properties that went fallow after their owners died or got too old to manage them.

It’s the kind of scene that, Mr. Saito’s critics argue, could be common across a Japan under degrowth policies.

But he has never really believed that society needs to return to some idyllic, agrarian lifestyle.

“I’m not saying let’s go back to the Edo period,” he said, referring to the feudal era when the country was largely closed to the rest of the world.

His vision for the future is one in which people — less consumed by their endless pursuit of growth for growth’s sake — have the leisure time to spend a workday pursuing new interests, as he does with farming.

On a recent day, Mr. Saito spent several hours working alongside the organic farm’s owners, Shoko Nakano and her husband, Sho Nakano. Local residents popped in to buy vegetables from a shack built out of recycled materials, while an enormous sow snuffled in the heather beside a vegetable garden.

After Mr. Saito spent a few hours driving bamboo stakes into a field with a heavy wooden mallet, Ms. Nakano asked him if he felt energized by his experience wielding a symbol of the proletariat.

Mr. Saito laughed. “I’m definitely bourgeois,” he said.