In the 19th century, Boston was home to the American Industrial Revolution, the abolition movement, and the women’s rights movement. It was known as the “Athens of America,” and its population grew so fast that we needed to fill in the South Cove, South End, Back Bay, and parts of East Boston and South Boston.

What followed? From the early 1920s, when a national recession prompted the region’s industrial base to shutter and chase lower-cost labor, through the 1980s, a decade that ended with the embarrassment of the Charles Stuart murder-suicide, Boston was an urban basket case. In 1959, Boston’s bonds were rated at just above junk level. In 1950, the population of the city was over 800,000; by 1980, the city had lost nearly 300,000 people. As recently as 1982, the Brookings Institute declared Boston the most blighted big city in America.

As a land-use attorney, I talk regularly with real estate development clients grappling with the question of whether to continue to invest in Boston. After all, the future for the city seems shakier than it has in a generation, with Boston wrestling with housing, transportation, climate change, and equity crises. Hybrid work has emptied downtown, shuttered businesses that cater to workers, and threatened the city’s tax base.

To answer my clients’ questions, I felt I had to better understand why Boston triumphed for nearly a century, then stagnated for seven decades, only to succeed again over the last three decades. Starting in 2017, I read every book I could find about Boston history. I found nearly 300 of them by scouring used bookstores and hunting them on the web. I was looking for patterns and practices that have coincided with urban success and failure, which I describe in a forthcoming 800-page book, “A History of Boston.”

What have I learned?

Colonial and Federal-period Boston, as the financial capital of New England, enjoyed a maritime (fishing and whaling) / agrarian (farming) / mercantile (trade) economy. The region’s whaling ships spanned the globe and provided the oil that lit cities like London, its trading ships called upon ports in China, India, Russia, and (sadly) Africa (as part of the triangular trade in enslaved persons). Yet, by the 1840s, Boston’s tripartite economy was in steep decline, with farmers leaving the region for the fertile grounds of the Ohio River Valley and the Erie Canal funneling the nation’s products to New York instead of Boston.

But Boston proved itself resilient. A city of ideas, Boston in the 19th century embraced religious reform (Unitarianism), philosophical exchange (Transcendentalism), educational advances (led by pioneers like Horace Mann), medical innovation (Susan Dimock, Dorothea Dix), and prison restructuring. Famous as the center of the American abolition movement in the 1850s, Boston is less well known for the nation’s first civil rights movement, in the 1840s, when the city’s free Black community used all the tools that later became familiar during the 1950s and 1960s civil rights movement in the American South — civil disobedience, picketing, lobbying, leafletting, and litigation — to integrate places of worship and employment and, finally in 1851, the schools (even if that integration, as we learned in the 1970s, proved short-lived). Boston’s willingness in the 19th century to accept new and diverse ideas — ideas that challenged orthodoxies — laid the foundation for the innovation that drove the American Industrial Revolution and made New England a manufacturing powerhouse.

Yet when those mills started closing in the 1920s, after the 1920 recession and the shutting of the country’s borders to immigrants in 1924, Boston did not reform its economy. For seven decades, Boston did not innovate. Instead, Boston retreated to tribalism — white vs. Black, Protestant vs. Catholic — shutting out diverse people and ideas. “Banned in Boston” was a real thing; Boston had a city censor into the 1980s! Rather than investing in the city as it had in the 19th century, Boston tore down its dense neighborhoods in the name of urban renewal and highway building, letting people and jobs move to the suburbs.

It turns out that for cities to evolve and thrive, innovation must be actively cultivated by creating inviting spaces that bring together diverse people with diverse ideas. The three Ds — density, diversity, and good urban design — are the foundation for city building. Only when Boston reembraced these principles, starting with the backlash against urban renewal in the 1960s and finally more broadly in the 1990s, did the city finally reinvent itself with its knowledge-based economy of the last three decades.

These principles are our best local response to the concurrent crises we face today. Building dense housing in the city and near public transportation addresses our housing shortage and gets people out of their cars, improving our roadways and reducing carbon emissions — and creates the conditions in which we have learned from our history that innovation thrives.

The pandemic shook our faith in urbanism. Yet the vaccines that helped us get past the worst of COVID-19 were all created in urban areas, including Moderna’s here in Greater Boston, and some of the best hospitals to treat the effects of the pandemic were in the city. Remote working sure is convenient, but innovation depends on bringing people together again. We knew these basic principles of urbanism in the 19th century (even if that term was not known then), we forgot them through much of the 20th century, and we’ll need to remember them in the 21st.

Daniel P. Dain, a partner in the Boston law firm of Dain, Torpy, Le Ray, Wiest & Garner, is the author of “A History of Boston,” due out on Sept. 19. He will be discussing the book at the Cambridge Public Library at 6 pm on Sept. 18, in an event sponsored by the Harvard Book Store.