"I went to the woods because I wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach, and not, when I came to die, discover that I had not lived."

On the fourth of July, 1845, a month and a half after Sir John Franklin set out from London with the ships Erebus and Terror to find the Northwest Passage, Henry David Thoreau set out from the family home in Concord, Massachusetts, to take up residence at nearby Walden Pond to find himself. He was not yet 28. He had a degree from HarvardCollege, he had tried teaching and failed, and he possessed some skill in surveying. He had almost no money, but he had friends, by far the most valuable of whom was his neighbor Ralph Waldo Emerson. Thoreau had built himself a 10- by 15-foot cabin with secondhand lumber on shoreline property at Walden owned by Emerson.

Thoreau lived at the pond for two years, two months and two days. His idea was to conduct an experiment in simple living, to lead a life according to nature and to determine the real necessities of life. "It would be some advantage," he wrote, "to live a primitive and frontier life, though in the midst of an outward civilization." Walden, published 150 years ago this month, is Thoreau's report on this modest—almost backyard—experiment in getting back to basics.

The book is conceded to be a masterpiece and figures on every list of essentially American books, but in this day and age, we may legitimately wonder whether Thoreau's experiment in plain living has any meaning at all for a generation weaned on cellphones, the Internet and Nintendo.

I know that when I first tried to read Walden, at age 15, Thoreau did not speak to my condition or to my life goals, which were, at the time, to get a car and meet girls. Nor is he exactly easy to read: his sentences are not always as clear as "The mass of men lead lives of quiet desperation," and he can irritate a reader when he turns on him: "It is very evident what mean and sneaking lives many of you live." Finding this on page three of Walden, I put the book down.

I did not wake up to Thoreau until I was 40, though I had by then been teaching him, in an empty, pro forma way, for 15 years. One day, I found myself rereading Thoreau's essay "Walking," in which he tells about going to see two panoramas, one of the RhineRiver and its storied castles, the other of the Mississippi. (A panorama is a long roll of painted canvas slowly wound from one roll to another across a stage before an audience, a sort of precursor of the movies.) Looking at the unadorned Mississippi panorama, he writes, "I saw that this was a Rhine stream of a different kind; that the foundations of castles were yet to be laid, and the famous bridges were yet to be thrown over the stream; and I felt that this was the Heroic Age itself, though we know it not, for the hero is commonly the simplest and obscurest of men."

Though I had read and taught this essay many times, I now saw it, for the first time, not as a text, but as a truth. Just as Thoreau was startled to discover that his era, the 1840s and 1850s, was as heroic as the Middle Ages, so he had startled me into the realization that I walk in the same natural world that Thoreau did. Like that essay, Walden is a self-help book, perhaps the ultimate self-help book, urging us to show up for our own lives, to have the courage to find our own convictions and to try to live them out.

As soon as I understood that Thoreau was talking directly to me, the mythic Thoreau, the hermit of Walden Pond, the echo of Emerson, the isolated and lonely figure from America's rural past vanished from view. In his place stood a writer of immense humanity, vitality and humor.

Thoreau is a man of terrific intensity. We become aware of this through his passionate insistence on seeing—a "habit of attention" he once said he possessed to such a degree that it fatigued his senses. We all look at the same things, but some see more than others. "A single gentle rain," Thoreau observes in his chapter on spring, "makes the grass many shades greener."

Allied to his acuity of sight, his granting to every object a "separate intention of the eye," is Thoreau's great learning. Yes, he required a four-hour walk every day to keep in good spirits. But he also spent four hours or more every day at his desk, reading and writing. He read Virgil, Goethe, Linnaeus, Darwin and Ruskin. He read travel books, the classics, botany, zoology, philosophy, politics and economics. He was, in critic Edward Davidson's nice phrase, a chain reader. Like Pliny the Elder, who read or had himself read to every leisure hour, even in the bath, Thoreau apparently found no book so bad it couldn't be used in some fashion.

During his first year at Walden Pond, Thoreau cultivated about two and a half acres of Emerson's land, planting and tending potatoes, corn, peas, turnips and, chiefly, beans. What he didn't eat he sold. Contrary to popular belief, he went to town frequently, entertained visitors at the cabin and once even hosted a large picnic there for an abolition society. But mostly he worked at his desk, where he accomplished a great deal of writing. Drawing on a two-week-long trip to Mount Katahdin in Maine and his brief arrest in Concord for failing to pay his poll tax, he wrote essays on "Katahdin" and "Civil Disobedience" (which remains the preeminent American statement of the primacy of individual conscience). He wrote one book (A Week on the Concord and Merrimack Rivers) and the draft of a second (Walden).

In his writings, and in Walden above all, Thoreau forged a thought-out way of life, a philosophy that insists that the individual turn not to the state, not to the gods, not to society, or even to history for a guide to life, but to nature and the self. But this turn to nature and the self should not be confused with selfishness. It is not the final destination but only the starting point of the examined life. Thoreau's social side is everywhere in Walden. "I had more visitors while I lived in the woods," he says in the chapter "Visitors," "than at any other period of my life."

Thoreau's second great achievement is one he shares with Emerson and other American Transcendentalists: the articulation of the social imperatives of their movement. If I wish to be free, the Transcendalists argued, then all must wish to be free, and none may be denied freedom. In the formulation of the African-American writer and leader Frederick Douglass, "there is not a man beneath the canopy of heaven who does not know that slavery is wrong, for him."

Thoreau's activism led him to make speeches and organize meetings to protest slavery, to work for the Underground Railroad, to defend the abolitionist John Brown and help get one of his men to Canada, and to write "Civil Disobedience." Walden is full of incisive social and economic analysis. "I cannot believe that our factory system is the best mode by which men may get clothing.... The principal object is, not that mankind may be well and honestly clad, but, unquestionably, that the corporations may be enriched."

Thoreau's third great achievement is that he first articulated America's conservation ethic. When Thoreau said, famously, "in Wildness is the preservation of the world," he means the preservation of civilization too. "Our village life would stagnate," he wrote in Walden," if it were not for the unexplored forests and meadows which surround it. We need the tonic of wildness."

Walden was published on August 9, 1854, to mostly good reviews, and it developed a small but steady following. It sold roughly 300 copies a year for the next 15 years. The naturalist John Muir read it, as did poets William Butler Yeats and Robert Frost. After a brief dip in popularity in the 1870s and early 1880s, the book began the steady climb that carried it through the 20th century and that shows no signs of slowing.

Thoreau's Walden speaks to our modern condition because it is mostly right about the big things. Open the book anywhere: One should beware of all enterprises that require new clothes. A person is rich in proportion to the number of things he can afford to let alone. Morning does bring back the heroic ages. The Universe is wider than our views of it. Heaven is under our feet as well as over our heads. The sun is but a morning star.

One hundred fifty years after its publication, Walden also remains a practical, usable manual on how to lead a good, just life. It offers readers an ethical view of life that begins in self-rule and ends in public and social commitment to the next generation. Gandhi picked this idea up from Thoreau, among others, and he put it with admirable pith and sinew. "[Real home rule] is self-rule or self-control.... If man will only realize that it is unmanly to obey laws that are unjust, no man's tyranny will enslave him," Gandhi wrote in Indian Home Rule. At its core, Walden is about the project of personal freedom, self-emancipation, which is where all pursuits of freedom must start.

Thoreau left Walden Pond in 1847, saying he had "several more lives to live." He stayed for a while with the Emersons; he traveled to Maine and Cape Cod. He read Darwin's Origin of Species and felt that it squared with his own observations. He spent years gathering material for a never-realized "Calendar of Concord," an ambitious design to record even the smallest natural fact about Concord and thus make of it a microcosm of nature as a whole. One day in 1860 while he was out counting the rings of recently cut trees, he caught a cold, which turned to bronchitis and then aggravated an existing case of tuberculosis. Thoreau died on May 6, 1862, as the Civil War was raging. He was 44.

Sir John Franklin not only never found the Northwest Passage, he never returned to England. His wife sent out expedition after expedition to find him. "Is Franklin the only man who is lost, that his wife should be so earnest to find him?" Thoreau asks in the conclusion of Walden. Then he gives us his final bit of advice. "Be rather the Mungo Park, the Lewis and Clark and Frobisher, of your own streams and oceans. Explore your own higher latitudes." That task is just as hard now as it was in 1854, and Henry Thoreau is still one of the best guides around.