The sun was sinking low over Long Island Sound as Stephanie Kelton, wearing the bright red suit jacket she had donned to give a virtual guest lecture to university students in London that morning, perched before a pillow fort she had constructed atop the heavy wooden desk in her home office.The setup was meant to keep out noise as she recorded the podcast she co-hosts, a MarketWatch production called the “Best New Ideas in Money.” The room was hushed except for Ms. Kelton, who bantered energetically with the producers she was hearing through noise-blocking headphones, sang a Terri Gibbs song and made occasional edits to the script. At one point, she muttered, “That sounds like Stephanie.”What Stephanie Kelton sounds like, circa early 2022, is the star architect of a movement that is on something of a victory lap. A victory lap with an asterisk.Ms. Kelton, 52, is the most familiar public face of Modern Monetary Theory, which posits that if a government controls its own currency and needs money — to make sure its citizens have food and places to live when, say, a global pandemic pushes many out of work — it can just print it, as long as its economy has the ability to churn out the needed goods and services.In the M.M.T. view of the world, “How will you pay for it?” is a vapid policy question. Real-world resources and political priorities determine how much lawmakers can and should spend.It is an idea that was forged, and put to something of a test, during a low-inflation era.When Ms. Kelton’s book, “The Deficit Myth,” was published in June 2020 and shot onto best seller lists, inflation had been weak for decades and had dropped below 1 percent as consumers retrenched in the pandemic. The government had begun to spend rapidly to try to prop up flailing households.When Ms. Kelton appeared on a Bloomberg podcast episode, “How M.M.T. Won the Fiscal Policy Debate,” in early 2021, inflation had bounced back to around 2 percent.But by a chilly January afternoon, as ducks flew over the frosty estuary outside Ms. Kelton’s house near Stony Brook University, where she teaches, inflation had rocketed up to 7 percent. The government’s debt pile has exploded to $30 trillion, up from about $10 trillion at the start of the 2008 downturn and $5 trillion in the mid-1990s.The good news: The government has had no trouble selling bonds to fund its spending, contrary to the direst projections of deficit scolds.The bad news: Some economists blame big spending in the pandemic for today’s rapid price increases. The government will release fresh Consumer Price Index data this week, and it is expected to show inflation running at its fastest pace since 1982.And that may be why Ms. Kelton, and the movement she has come to represent, now seem anxious to control the narrative. The pandemic spending wasn’t entirely consistent with M.M.T principles, they say — it wasn’t assessed carefully for its inflationary effects as it was being drawn up, because it was crisis policy. But the situation has underlined how hard it is to know just where the economy’s constraints lay, and how difficult it is to fix things once you run into them.Last summer, Ms. Kelton called inflation a temporary sign of “growing pains.” By the fall, she painted it as a good problem to solve, compared with a continued weak economy. As it lingers, she has argued that diagnosing what is causing it is key.“Can we blame ‘MMT’ for the run-up in inflation?” she tweeted rhetorically last month, just hours before her podcast recording.Understand Inflation in the U.S.Inflation 101: What is inflation, why is it up and whom does it hurt? Our guide explains it all.Your Questions, Answered: We asked readers to send questions about inflation. Top experts and economists weighed in.What’s to Blame: Did the stimulus cause prices to rise? Or did pandemic lockdowns and shortages lead to inflation? A debate is heating up in Washington.Supply Chain’s Role: A key factor in rising inflation is the continuing turmoil in the global supply chain. Here’s how the crisis unfolded.“Of course not.”Emon Hassan for The New York TimesThe economy is the limitTo understand how M.M.T. fits in with other dominant ways of thinking, it’s helpful to take a trip to the beach.In economics, there’s a school of thought sometimes called “freshwater.” It’s the set of ideas that became popular at inland universities in the 1970s, when they began to embrace rational markets and limited government intervention to fight recessions. There’s also “saltwater” thinking, an updated version of Keynesianism that argues that the government occasionally needs to jump-start the economy. It has traditionally been championed in the Ivy League and other top-ranked schools on the coasts.You might call the school of thought Ms. Kelton is popularizing, from a bay that feeds into the East River, brackish economics.M.M.T. theorists argue that society should feel capable of spending to achieve its goals to the extent that there are resources available to fulfill them. Deficit spending need not be constrained to recessions, even theoretically. Want to build a road? No problem, so long as you have asphalt and construction workers. Want to feed children free lunches? Also not a problem, so long as you have the food and the cafeteria workers.What became Modern Monetary Theory began to percolate among a small group of academics when Ms. Kelton, a former military brat and one-time furniture saleswoman, was a graduate student.She had a gap period between graduating with a bachelor’s degree from California State University, Sacramento and attending Cambridge University on a Rotary scholarship, and her college economics professor recommended that she spend the time studying with L. Randall Wray, an early pioneer in the set of ideas.They hit it off. She remained in Mr. Wray’s circle, and he — and Warren Mosler, a hedge fund manager who had written a book on what we get wrong about money — convinced her that the way America understood cash, revenues and budgeting was all backward.Ms. Kelton earned her doctorate at The New School, long a booster of out-of-mainstream economic thinking, and went on to teach at the University of Missouri-Kansas City. She, Mr. Wray, who was there at the time, and their colleagues mentored doctoral students and began to write academic papers on the new way of thinking.But academic missives reached only a small circle of readers. After the 2008 financial crisis punched a hole in the economy that would take more than a decade to fill, Ms. Kelton and her colleagues, invigorated with a new urgency, began a blog called “New Economic Perspectives.” It was a bare bones white, red and black layout, using a standard WordPress template, that served as a place for M.M.T. writers to make their case (and, in its early days, featured a #Occupy[YourCityHere] tab).The theory picked up some fervent followers but limited popular acceptance, charitably, and outright derision, uncharitably. Mainstream economists panned it as overly simplistic. Many were confused about what it was arguing.“I have heard pretty extreme claims attributed to that framework and I don’t know whether that’s fair or not,” Jerome H. Powell, the Fed chair, said in 2019. “The idea that deficits don’t matter for countries that can borrow in their own currency is just wrong.”Ms. Kelton kept the faith. She and her colleagues held conferences, including one in 2018 at The New School where she gave a lecture on “mainstreaming M.M.T.”Rohan Grey organized the conference and a media reception afterward at an Irish pub (“‘Shades of Green,’ monetary pun intended,” he said). It was attended by organizers, academics, “lay people” and lots of journalists. At the happy hour — which lasted until 1 a.m. — Ms. Kelton was mobbed when she walked in the door. “She was already on her way to super celebrity status at that point,” said Mr. Grey, an assistant professor at Willamette Law.When she gave presentations on her ideas, Ms. Kelton would occasionally display a quote often attributed to Mahatma Gandhi: “First they ignore you, then they laugh at you, then they fight you. Then you win.”And her star was rising more broadly. She advised Bernie Sanders’ presidential campaigns in 2016 and 2020, getting to know the Vermont senator. He never fully publicly embraced M.M.T., but he nevertheless advanced policies — like Medicare for All — that reflected its ideals.She amassed a following of tens of thousands, later growing to 140,000, on Twitter. Her first handle, @deficitowl, prompted ardent fans to gift her wise bird figurines, some of which are still on display in her home office. She cultivated a small coterie of prominent journalists who were interested in the idea, most notably Joe Weisenthal at Bloomberg. She signed a book deal. She was regularly talking to Democratic lawmakers, sometimes in groups.Her idea percolated through Washington’s media and liberal policy circles. Mainstream economic predictions that huge debt loads would come back to haunt nations like Japan had not played out, the anemic rebound from 2008 had scarred society and called the size of the crisis response into question. Ms. Kelton and her colleagues were ensuring that their theory on benign deficits was an ever-present feature of the blossoming debate.Then the pandemic hit, and suddenly the theoretical question of just how much the government could spend before it ran into limits faced a real-world experiment.The $1.9 Trillion FloorWithout thinking about paying for it, Donald J. Trump’s government quickly passed a $2.3 trillion relief package in late March 2020. In December, it followed that up with another $900 billion. President Biden took office in early 2021, and promptly added $1.9 trillion more.Inflation F.A.Q.Card 1 of 6What is inflation? More