NEW YORK — When she was in her 40s, Gretchen Rubin tackled happiness as a project because that’s how Rubin rolls. To be clear, she was happy. She wanted to be happier.

Her quest, resulting in the No. 1 bestseller “The Happiness Project,” inspired people — who are we kidding? women — to shed bad practices and develop far better ones.

The 2009 book launched Gretchen Rubin Inc., which encompasses more titles, book sales of 3.5 million, a community-sourced app (Better ) and a podcast (Happier) that has been downloaded 70 million times.

Friends warned her that the title was terrible, she recalls, and the book wouldn’t work because “either you’re so boring nobody cares, or you’re so idiosyncratic no one will identify with you, both of which are true.”

Actually, only the latter is true.

Rubin, 53, is a former editor in chief of the Yale Law Journal and clerk for Supreme Court Justice Sandra O’Connnor. But after only a few years, she quit the law with zero remorse and became a thought leader on inner growth. Rubin lives in a sumptuous Upper East Side triplex, with her husband, Jamie, a private equity fund manager; a daughter (with another in college); and a cockapoo.

She’s in four book groups. For her happiness project, Rubin compiled 92 goals. Her home is Architectural Digest gorgeous. And beyond tidy.

Rubin isn’t a Type A. She’s a Type A Plus.

You know the type. Surgeons who bake heirloom-grain bread for school events. Marathoners who run at 5 a.m. listening to Cervantes in Spanish. Mothers of four who are thinner than they were in grad school. On screen, they’re portrayed by Reese Witherspoon and Jennifer Garner.

Now imagine that Type A Plus person telling you how to live, through her blog, podcast and books. Such gumption makes Rubin a polarizing figure. Many friends and colleagues of mine groan when they hear her name. Yet, in the past decade, Rubin’s become a popular savant on self-improvement. That’s the core of the Rubin paradox: Why do people seek advice from someone who appears to have achieved so much so effortlessly — and who flits from topic to topic with the supreme confidence of an authority on all of them?

Her background is in law, not psychology, but two years ago Rubin publishedThe Four Tendencies: The Indispensable Personality Profiles That Reveal How to Make Your Life Better (and Other People’s Lives Better, Too).” She calls her theory “the most major insight I felt I’ve had in my whole life.”

Rubin, of course, is an Upholder (the others are Questioner, Obliger and Rebel). An Upholder honors both outer and inner expectations — while other people maintain lesser habits like cake and sloth.

Reviews were not kind. “Breezy but unconvincing work of pop psychology,” wrote Publishers Weekly. “Analysis lacks psychological or scientific grounding, and it can lead to questionable conclusions,” Kirkus noted, calling it “ultimately insubstantial.”

Most likely, Rubin did not read these. She doesn’t look at reviews and profiles of herself, due to “negativity bias,” that adverse comments are more memorable than positive ones.

In writing “The Four Tendencies,” Rubin didn’t feel the need to consult psychologists. “Res ipsa loquitur. The thing speaks for itself,” she says, sitting before one of two working fireplaces in her book-lined living room. “I really see myself more like someone like Samuel Johnson,” the 18th century man of letters, “like not that I would compare myself to Samuel Johnson but like someone whose observations are penetrating because they’re true.”

Rubin never uses the term “self-help,” though her books are labeled as such. In many ways, she is filling the be-a-better-you space long occupied by glossy women’s magazines when they were thriving: fast fixes, quizzes, labels and comfort. This year, Rubin launched two online courses: one on happiness, the other on her four tendencies. About 10,000 people enrolled.

While researching “Tendencies,” she jotted down notes for “Outer Order, Inner Calm,” published this week, a collection of more than 120 random tips and aphorisms, though mainly it’s about clutter.

Yes, clutter.

Best to address the Kondo in the room: In a KonMari method-mad world, do we need another book on clutter? And, isn’t producing another book on clutter a further act of clutter?

“If there’s a book that’s been hugely successful, it’s usually a sign that people are really interested in the subject,” says Rubin, a passionate fan of the tidying Netflix sprite. Marie Kondo has a specific method and order, Rubin says, whereas “I feel there’s no one way to clear clutter.”

Kondo doesn’t like a lot of books. Rubin’s home is filled with them. Kondo asks if a thing “sparks joy.” Rubin prefers asking “Does this energize me?”

Why are we experiencing this anti-clutter moment? “The world just feels very noisy and overwhelming,” Rubin says. “One way to bring down the noise in your head is to bring down the noise in your environment and to make your life easier and calmer.”

This coming from a woman who cleans friends’ closets for fun. “I’ll spend as long as they let me,” she says. “Usually, they kick me out.”

She wants to help. She needs to help. Her younger sister Elizabeth Craft, a TV writer and producer who co-hosts Rubin’s podcast, wryly dubbed her a “happiness bully.”

“You’re on the fringe. You’re not like normal human beings,” Craft once said to her. “Other people don’t decide they’re never eating cheese again — and stop.”

“You are right,” Craft recalls Rubin responding. “I really am different.”

Sharon Greenberger, CEO of the YMCA of Greater New York, and a friend from middle school in Kansas City, Mo., says “she had innate skills we didn’t have. I stand in awe of her. It’s like being around the sun.”

I was shocked to learn that Rubin gets up as late as 6 a.m.; less so that she maintains the same schedule on weekends and vacations. A blown deadline is preposterous. “She usually delivers early,” says her agent, Christy Fletcher.

“I’m very struck by the range of people who identify with her. She lives in New York and has a life that is somewhat removed from many people,” Fletcher says. “Yet the people who write to her are quite impassioned. There’s something universal about what she’s writing about, the ideas and the experience, that becomes equalizing.”

Rubin’s suggestions tend to be rooted in common sense. She rarely writes about money and celebrity. Fans may not realize that when she mentions “my father-in-law Bob,” she’s referring to former Clinton Treasury secretary Robert Rubin. She calls herself a classic “underbuyer,” a master of thrift, someone who doesn’t have enough gloves in the house when winter arrives — even if that house is decorated by the legendary late designer Mario Buatta, with walls drenched in custom paint and museum-quality wallpaper in the formal dining room.

She loves to share — her diet, her lists — which endears her to acolytes. “She sort of lives out loud,” says her friend Julia Bator. “She’s very narrative in her podcast and in her work.”

Nor is Rubin afraid to abandon something or admit there are whole areas that don’t interest her, like music. She tried meditation twice. “Got nothing out of it,” she says.

Rubin is fit, slender. “I gave up carbs,” Rubin says. And sugar. And fat. She rarely drinks. Any bad habits? “I am a terrible hair twister.”

No, Gretchen, that is not a bad habit.

“I feel like I must have bad habits,” she says. Pause. “I’m really bad about putting my clothes away.”

Rubin tried conquering her dislike of driving, even taking refresher classes, though she lives in Manhattan, where driving isn’t necessary. “I’ve backslid in my fear. I feel bad about that. I should expect more of myself for that.”

Part of Rubin’s appeal is her Midwestern roots. Her voice has a hard flatness, free of affectation. She gets excited about almost everything.

Order becomes her. The Rubin oeuvre is salted with lists: The (12) Habits Manifesto, the Nine Promises, her 12 Commandments.

She compiled 19 resolutions for 2019, including reading Proust, developing merchandise and learning to play the ukulele.

Each year is blessed with a theme; 2019’s is growth. “How do we take everything to the next level?” Rubin asks. “Like, how do I even think about that?”

Must our lives always be under construction, each day tasked with becoming ordered, calmer, better, happier, more? Can’t we simply be?

“That’s one of the central tensions within the subject of happiness,” Rubin says. “It’s an absolutely crucial thing to think about because, on the one hand, you want to accept yourself, and on the other hand, you want to expect more from yourself.”

She wants to develop a television series, and write more books: an expansion of her Secrets of Adulthood blog posts and one on the “Ten Senses,” another Rubin theory.

Yes, she’s identified five additional ones.

“I’m interested in profoundly experiencing reality,” she says. “And that’s through the body, in large part.”

She’s promoting her clutter book “Outer Order” in more than 25 cities. The tour presents an opportunity to fulfill another resolution, more reading instead of watching HGTV in her hotel room. In preparation, Rubin amassed about four dozen volumes, which neatly fill two entire shelves.