“A Journal,” as Henry David Thoreau defined it in an entry from July 13, 1852, is “a book that shall contain a record of all your joy—your extacy.” Thoreau’s archaically spelled “extacy” infuses the Journal with a mystical charge. It hints at his almost fanatical practice of self-documentation, spotlighted in “This Ever New Self: Thoreau and His Journal,” a joint exhibition between the Morgan Library & Museum and the Concord Museum in Massachusetts, on display at the Morgan through September 10.

For Thoreau, “extacy” lay not in organized religion but in his private thoughts and their crisscrossings on paper, as we see in “This Ever New Self.” His distinctly American Journal contains multitudes—it is dazzling and self-aggrandizing compared to the quiet assiduity of, say, English diarists. For Dorothy Wordsworth, it was enough to record that the weather was fair, a touch hazy, her headache greatly relieved since yesterday, the wild strawberries just peeking through. Thoreau had to make the Journal something more than it was for his own purposes as an American and a New Englander.

He takes what might be called a watercolorist’s approach—setting out early in the morning, scribbling his observations in pencil on slips of paper now mostly lost, and then expanding on them in his beloved Journal in the privacy of his chamber, with ink and quill. The chilly air of New England may have had something to do with his method; perhaps the compressed and vigorous notes taken out of doors could only loosen up and unfurl by the warmth of the fireplace.

Thoreau, who kept his Journal from the time he graduated Harvard at twenty in 1837 until a few months before his death in 1862, maintained it as rigorously as he conducted land surveys. He was methodical, vigorous. The surveyor’s maps and other supplementary materials on display at the Morgan make that clear, helpfully sketching a link between the journals and Thoreau’s life as a naturalist and generalist. The curators make little distinction between his verbal and visual writing, lending the exhibition greater fullness and diversity. Interspersed among the journals are maps, weather logs, pressed plants from Thoreau’s herbarium, and a floor plan he drew of his house.

Nor does it make sense to separate Thoreau’s writing from daily, practical things like his compass, his ruler, his walking stick, and his spyglass, all of which are on view. Thoreau even likened himself to a farmer, writing in 1852: “Having by chance recorded a few disconnected thoughts and then brought them into juxtaposition—they suggest a whole new field in which it was possible to labor & to think.” Labor was always on Thoreau’s mind. As an American, and as a New Englander, Thoreau surely understood that idle hands are the devil’s playthings. For Thoreau, writing down his “few disconnected thoughts” outdoors was not just a pleasant way to pass the time—it was honest work.

And so the pencil became a tool. (In fact, Thoreau grew up making pencils for his father’s business and held the humble objects in reverence.) Not for nothing did Thoreau keep weather logs and record the life cycles of various plants. These accounts are so meticulous, the curators explain, that climate scientists are now consulting them as primary sources. Thoreau, an ardent admirer of Linnaeus (and an anxious Harvard grad), thought it necessary to bring discipline and order to the very practice of keeping a diary.

What’s remarkable is how far Thoreau went in treating the Journal as a workplace. The “field” of his diary did not fail him, but turned out to yield fruit. Walden, which grew out of journal entries from July 1845 to the fall of 1847,is a case in point. Thoreau’s great work sprang like a plant from the dark, rich soil of ten thousand journal entries. A scrap of paper where Thoreau penned its famous opening paragraph sits inconspicuously among the journals, hardly distinguishable from them. Thoreau’s speeches, too, were based in some cases directly on past journal entries, as tear-outs from his early period attest. His works, in other words, do not dwell apart from the work of keeping a journal.

Thoreau’s Journal is obscenely thorough, and it’s no surprise that it’s taken scholars one hundred fifty years to get around to publishing it in full. (“This Ever New Self” coincides with Thoreau’s bicentenary and the ongoing publication of his journals by Princeton University Press.) It seems indisputable that Thoreau meant for them to be read by posterity. For one thing, we see in “This Ever New Self” that he frequently rewrote words he deemed illegible, indicating that he had some audience in mind. (Caveat emptor: Viewers attempting to decipher the diaries will find themselves maddened by Thoreau’s execrable penmanship—and grateful to the Morgan for the plentiful transcriptions included throughout the exhibition, though these are not always complete.) There is no scientific way to prove intention, but all signs point to a more-than-dim awareness of future readers on Thoreau’s part. The curators are justified in calling the Journal “his most essential work of art” and presenting it as such.

Thoreau’s grandiose claims about the Journal and his evident desire that posterity would read it (as we now indeed are doing) in no way contravene the lightness of touch and brilliance of observation we find on display. We even witness a demureness in the latter entries absent in the early ones; Thoreau, it seems, thought better of his grandiose strain and shifted into a more modest register. It is a credit to the Morgan’s curators that they have ably captured these subtle shifts, given the distinct disadvantage of book exhibitions, which only allow a tiny fraction of the material actually to be seen. “This Ever New Self” presents a snapshot of a voluminous journal, which scholars will take years to sift through properly, offering viewers highlights and an inkling of the man who went into the woods and came back with a book that changed the country.

In a poignant letter on display, written to Isaiah Thornton Williams on March 14, 1842, Thoreau wonders at his brother John’s death from the “slightest apparent cause—an insignificant cut on his finger, which gave him no pain, and was more than a week old.” “Perhaps,” he muses, “we never assign the sufficient cause for anything, though it undoubtedly exists.” His insistent curiosity, about himself and about nature, this desire to get to the bottom of things, made him an incurable diarist. It wasn’t just on the occasion of a death in the family—not a leaf could be looked at nor a stone turned over without there being ripe material for self-reflection. In an age of inattention, this savant-like capacity for observation makes Thoreau interesting to reread and worth reappraising at “This Ever New Self.”