A FEW WEEKS AGO, I opened the door of my mailbox to find a quiet bombshell waiting. It weighed two pounds and came wrapped in brown paper, stamped “media mail,” that, when torn away, revealed the daguerreotype portrait of a man rumpled, whiskery, and sad-eyed, six years from his tubercular death. This is the cover of Henry David Thoreau: A Life, a new, 640-page biography by the literary scholar Laura Dassow Walls.

Thoreau has always been too much for some people. How to make sense of a man who not only penned one of the most famous works of American literature, but was also a cutting-edge scientist, a visionary political theorist, a highly disciplined land surveyor, a radical abolitionist, and an inventor who helped make J. Thoreau & Co.’s pencils the most widely sought in the United States? He could be witty, abstruse, amenable, cryptic, and cutting, all in the course of an evening’s conversation. He has left thousands of pages of writing to us in books, essays, poems, maps, and notes; his journal alone runs to over two million words.

This is a lot for anyone to digest, and so skeptics and acolytes alike have tended to carve off whatever hunk of Thoreau’s life and work they are able to get their mouths around. There are those who see Thoreau as one of the earliest fathers of an American-style, wilderness-focused environmentalism, and those who are drawn to the Thoreau of “Civil Disobedience” (originally called “Resistance to Civil Government”) with its anarchistic rallying cry, “That government is best which governs not at all.” There are the disciples who claim Walden as their Bible, and the equally zealous scoffers who, as Walls writes, have given us a Thoreau “chilled into a misanthrope, prickly with spines, isolated as a hermit and nag.”

“[T]he Thoreau I sought was not in any book, and so I wrote this one,” Walls tell us in her introduction. Of course, there is already a long shelf of books concerned with Thoreau, including two canonical biographies: Walter Harding’s The Days of Henry Thoreau, originally published in 1965, and Robert D. Richardson Jr.’s Henry Thoreau: A Life of the Mind, from 1988. But, good as those earlier biographies are (and they are great), Walls’s Thoreau: A Life is the fullest, most insightful account of her subject yet. It chisels Thoreau free from the accreted layers of caricature that have long distorted his legacy. What makes Walls’s book work is an extraordinary fidelity to an enormous historical record that spans, in addition to Thoreau’s oeuvre, “the hundreds of books he annotated and the hundreds of charts and graphs where he pooled and organized his growing data,” a trove whose mastery only comes from a lifetime’s dedication.

It turns out that when you place Thoreau in historical context, misunderstanding evaporates in the heat of patient fact. For instance, Thoreau went to Harvard, a sin for which he’s still doing penance: his pedigree was one of the prime pieces of evidence the Ivy League–educated New Yorker writer Kathryn Schulz used to dismiss him as an out-of-touch elitist. And yet Walls tells us that, while at Harvard, Thoreau wore a green coat. Green was against the rules; Harvard men wore black. Thoreau, however, was too poor to afford two coats. After all, his family had sold their house and moved in with relatives to scrape together tuition, and Thoreau took time off from his studies to work as a teacher. So Harvard’s administration made for him a dress-code exception. Thoreau could wear green, though that green coat marked him as the token middling scholarship kid on work-study, rather more like those of us who, today, find ourselves crushed by towering student debt than a fortunate one whose veins run blue.

Walls does this throughout the book: turning to the historical record to dispel caricature and uncover the human truth. Thoreau as the stiff who never cracked a smile? Walls reveals all the irreverence — even sex jokes! — peppering his work, and recovers the anecdotes of his good-natured lightheartedness, especially around the young. (As he lay slowly dying from consumption, he asked his sister, Sophia, why the town’s children, who had so often been his companions on his walks, did not come to see him: “I love them as if they were my own,” he said; and though they were scared by the proximity of death, once Sophia invited them, come they did, to show him what they had found in Thoreau’s hometown of Concord’s fields, woods, and wetlands.) Thoreau the egocentric misanthrope who loved to lecture us all from high atop his soapbox? In an extraordinary five-page gloss on Walden, Walls acknowledges that “Thoreau’s anger and contempt can make us squirm today,” but she also shows that even his harshest polemics are lit by a profound empathy for passion and pain. Thoreau the lazy, privileged bum who squatted on Emerson’s land at Walden Pond? He daily hoed seven miles of beans that first year at Walden, on land he had to clear first, beside a house he built with his own hands, all while writing his first book.

But correcting the historical record, important as that may be, is only a small part of what Walls is up to. At the core of her book is the stunningly perceptive, deceptively simple insight that “[Thoreau’s] social activism and his defense of nature sprang from the same roots: he found society in nature, and nature he found everywhere, including the town center and the human heart.” Walden and “Civil Disobedience,” in Walls’s view, are of a piece, and Thoreau’s entire life, she contends, was spent in search of whatever it is that connects nature to society, the wild to the domestic, in one “community of life.”

As commonsensical as this claim may seem, it in fact flies in the face of decades’ worth of near-consensus that Thoreau was the champion of disconnection, of running away from a decadent, deadening society to a pure, solitary, natural idyll: to wilderness. This take on Thoreau really began in 1967, when the historian Roderick Frazier Nash published Wilderness and the American Mind. The Wilderness Act of 1964, which legally defined wilderness as “an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain,” had just become law, and Nash, who saw the preservation of wilderness as an unalloyed good, rooted the Act in a deep American history in which Thoreau was a key player: the first philosopher of American wilderness, as Nash put it, who “cut the channels in which a large portion of thought about wilderness subsequently flowed.”

The Nashian celebration of wilderness was a more-or-less stable consensus view until the late 1980s, when postcolonial critics rightly pointed out that imposing Western-style wilderness on places like India meant forcibly removing agrarian and pastoral communities from the land on which they had existed in a delicate balance, sometimes for generations, in order to retroactively render it “pure” and “untouched.” By 1990, the writer and environmentalist Barry Lopez, among others, brought this critique home to the United States. In his essay “Unbounded Wilderness,” Lopez argued that the very concept of wilderness “preserves a misleading and artificial distinction between ‘holy’ and ‘profane’ lands.” What we need instead is “another kind of relationship with the earth,” one of interconnection and mutual obligation — “informed reciprocity,” Lopez called it — that values “our farmsteads and the retreats of the wolverine” as well as “the land upon which our houses, our stores, and our buildings stand.”

Lopez’s critique of wilderness didn’t mention Thoreau, but one year later, Michael Pollan identified him, in the introduction to Second Nature: A Gardener’s Education, as the one who taught us all the “habit of seeing nature and culture as irreconcilably opposed” by leading “his descendants out of the garden” — that messy space where the human and the natural mix — and into the cathedral of wilderness. Pollan’s critique of Thoreau was cemented four years later when the eminent environmental historian William Cronon opened his essay “The Trouble with Wilderness; or, Getting Back to the Wrong Nature” by citing Thoreau’s famous declaration “in Wildness is the preservation of the world” as the watershed moment when Americans first turned toward an idealized, ahistorical wilderness — the ultimately harmful notion of land untrammeled by humans. And this is more or less where the critical consensus remains today: wilderness is a fantasy that disconnects us from the environments and social structures we all inhabit; wilderness erases the presence of humans (especially nonwhite, non-Western, non-wealthy ones); wilderness came from Thoreau.

Walls, however, swings our attention away from Thoreau as the philosopher of wilderness, to Thoreau the theorist of wildness: a shift in gravity that has the power to make us rethink not only our past, but also our future. She knits her study together carefully over the course of its 640 pages, setting the stitches that will become visible, lifelike patterns, deftly alternating bright threads of biography with strands of intellectual analysis. We learn, for instance, in the very beginning that Thoreau was obsessed by the question of “how to live the American Revolution not as dead history but as a living experience.” It makes sense that he would have this history on his mind: the opening volleys of the Revolution, “the shot heard round the world,” as Thoreau’s mentor Emerson put it, were fired in Concord, and aged veterans still could be seen tottering around the streets when Thoreau was a child; but the land the Revolution had won away from England seemed barren. Wherever Walls’s Thoreau looked, he saw that “inequality was rife, materialism was rampant, and the American economy was wholly dependent on slavery.” In such a country, what could freedom possibly mean?

One answer, the one we most associate Thoreau with today, was rejection, and as early as 1837 Thoreau was warning that there’s nothing free about the free market, which, even then, was gripping both politics and culture by the neck. The market, Thoreau intuited, shackles us to material accumulation, and if one would be free, one must walk away: “[A] man is rich in proportion to the number of things which he can afford to let alone,” he would write in Walden. This anticapitalist edge would never dull.

Yet it’s often forgotten that Thoreau embraced and engaged the world around him as often as he stomped off angrily to the woods in search of solitude. One of the many bright flickers that Walls uses to trace her constellation is the death of Thoreau’s brother John in 1842 from a freak shaving-accident-induced tetanus infection. It’s no secret that his brother’s death was devastating for Thoreau: he came down with a sympathetic case of phantom lockjaw and was ill for months after John’s passing. Walls discerns in John’s death Henry’s realization that he was terribly vulnerable to the pain of loss, a sensitivity that quickly turned to empathy, because loss is common to everything that lives. And so, facing the void, he reached out.

It’s this radical empathy that, as Walls puts it, “made him storm with the passionate and sleepless rage that powered his great writings of political protest”; it’s the obligation of radical empathy that, by the mid-1840s, made Thoreau among the most committed of white abolitionists in the United States: he conducted runaway slaves north on the Underground Railroad, shared a lecture stage with Sojourner Truth and William Lloyd Garrison, hosted Frederick Douglass when he came through Concord, and was the earliest public defender of John Brown after his botched raid on the federal arsenal at Harpers Ferry in 1859. Freedom, Thoreau had discovered, entailed an obligation to one’s neighbor.

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On July 4, 1845, Thoreau went to Walden Pond to begin his famous two-year experiment in deliberate living. Walls’s recounting of Thoreau’s Walden years is the longest part of the book, the white-hot intellectual center around which the convection current of her narrative churns. It’s the part where all the disparate pieces of the many Thoreaus — anarchist and scientist, nature-lover and neighbor, abolitionist and anticapitalist — fuse.

Walden made Thoreau, because it was there that Thoreau discovered nature. Of course, he had apprehended nature before, largely through the influence of Emerson’s thought, where physical nature was important mainly as a symbol of the Higher Power. But at Walden, he made his final key insight and distinguished himself as a thinker. Nature is not just symbolic of spirit: it is also the vibrant material foundation of everything, the thing joining mind to matter. Nature is you and me, the trees and the earth and all the little live things, the water in Walden as well as the wind whipping its surface. The free market, he knew, was as happy to sell human beings as land and offered no real path to liberty. Death had shown him that everyone is knotted together by loss, and that a commitment to freedom demands radical empathy. But it was Walden that expanded his moral aperture. “I wish to speak a word for Nature,” he would write in his famous essay, “Walking,” “for absolute freedom and wildness.”

Wildness. For Thoreau, who rarely used the word “wilderness” in his writing, wildness was the ungovernable spirit of life itself, inhering in everything and everyone. By the late 1850s, Walls argues that Thoreau was explicitly “imagining a turn to nature not as a return to primitivism, but as a contemporary renewal of the deep communal intertwining of nature and culture,” an embrace of life. This was wildness, the thing that ultimately led Thoreau to both abolition and environmentalism, the thing that joins “Civil Disobedience” to Walden, the individual to society, the human to the non-, and it is how he could proclaim that “in Wildness” — not wilderness — “is the preservation of the world.”

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Thoreau was in his intellectual prime when tuberculosis ultimately cut his life short. He was dedicated to the proposition that “attention to the natural environment confronted the root of all political evil” — exploitation — but he died before he could fully follow the course of his thinking through to its conclusions. “Thoreau’s life reads like a prelude to an alternative history that died in the cradle,” Walls writes, implying that there is no explicit tradition of Thoreauvianism in the United States. Yet Thoreau’s thinking has long quickened the pulse of radical American politics. Emma Goldman, who edited a journal called Mother Earth, called Thoreau “the greatest American anarchist” and found her inspiration partly in his example. Thoreau’s “Civil Disobedience” sprang up again in the liberatory movements of the 1960s, in the work of Martin Luther King Jr., and in activist Mario Savio’s famous speech on the steps of UC Berkeley’s Sproul Hall in 1964, which clearly echoes Thoreau’s figuring of the state as a machine in need of sabotage in “Civil Disobedience.” Thoreau inspired Edward Abbey, the cantankerous anarchist writer whose classic 1975 novel The Monkey Wrench Gang helped spark the radical environmental movement. More recently, he has influenced 21st-century intellectuals like Jedediah Purdy, Mark Greif, and Rebecca Solnit, the most Thoreauvian thinker and writer at work today. The alternative history to which Thoreau gave birth, then, didn’t die in the cradle — it has just remained in the shadows, and it’s time we reclaimed it.

With Henry David Thoreau: A Life, Walls puts the philosopher of wilderness to rest and reanimates a revolutionary Thoreau who remains defiantly in opposition to many of modernity’s gravest ills. What Walls ultimately returns to us is a Thoreauvian liberation ecology, dedicated to the defense of every living thing — exactly the legacy we need today. In This Changes Everything: Capitalism vs. The Climate, Naomi Klein wrote that we in the West must overcome “extractivism,” an ideology that sees the entire world and everything in it as just so much profit ready for the taking. Klein is right, but we’ve forgotten that she’s not the first to say so. “If you have built castles in the air,” Thoreau wrote at the end of Walden, “your work need not be lost; that is where they should be. Now put the foundations under them.” Walls’s remarkable book provides that rare thing: by giving us a solid footing in the past, she opens the possibility of a greener, freer future.

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Daegan Miller is a writer and landscape historian. His first book, This Radical Land: An American History of Natural Dissent, is out in February 2018.