Of course, scholarly books are still full of footnotes. The prototypic footnote is the source note, providing a citation for each proclamation in the text (early annotations were sometimes called “proofs”). These footnotes range from useful to pedantic, sometimes lending an air of authority, sometimes providing a map of the writer’s path. Legal writing in particular is rife with these footnotes, perhaps an acknowledgment that law is built on laws-past.

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But I champion another species of footnote: the wandering footnote. These digressive notes, seeing a sentence that some might consider complete, determine to hijack it with a new set of ever more tangential facts. In the wayward note, the bumps and curves of the author’s mind seem to be laid plain on the paper. I came of intellectual age hearing the author’s sotto voce asides in the philosophy essays I loved. I still recall footnotes that begin, enticingly, “Imagine that . . . ”; “Consider . . . ”; or even, in one of J. L. Austin’s famous thought experiments, “You have a donkey. . . . ” I had the feeling of being taken into confidence by a wise fellow during an erudite lecture, and being told something even more clever and lucid.

In fiction, I was spoiled by Nicholson Baker, whose novel “The Mezzanine” is largely footnotes — including a four-pager that starts: “And escalators are safe. . . . ” (A door has popped up unexpectedly and opened! I’m going in!) Smitten with the small type, I sought out the broader history of the footnote, covered to within a millimeter of its life in Anthony Grafton’s study “The Footnote: A Curious History” and Chuck Zerby’s “Devil’s Details: A History of Footnotes” (both are heavily footnoted). Grafton led me to such rollicking footnoters as Edward Gibbon, whose judgmental, conversational and explicatory notes in “The History of the Decline and Fall of the Roman Empire” lighten a weighty read. Such digressions and asides were so enthusiastically used in the 18th century that one satirist wrote a mock dissertation consisting entirely of footnotes. Pierre Bayle’s best seller “Historical and Critical Dictionary,” first published in the 1690s, charmingly used footnotes to point out the errors in the scholarship of others. I’ll take Grafton and Zerby’s word for it that John Hodgson’s mighty “History of Northumberland,” published a century and a half later, is at least worth flipping through for its footnotes on footnotes on footnotes, including one traversing 165 pages.

I have since found that attitudes on footnotes tend toward the hyperbolic. One scholarly writing handbook celebrates the “cartwheel” of the footnote, while Grafton compares the drone of the historian’s footnote to “the high whine of the dentist’s drill,” a sign that we are in the presence of professionals. Legal footnotes — the subject of particularly contentious debate — are “a mother lode, a vein of purest gold,” one judge gushed, while another legal writer called them “lead feet below the line.” Footnotes are “a rhapsodic grace note” in a master’s hands, a journalist wrote. More often, however, footnotes are slandered as “forbidding,” “unsightly,” “like a fungus”; and even, as one footnote-weary professor put it, a “subversive breed of mice.” I have come across more than one author who chose “excrescence” to describe footnotes. Noël Coward reputedly said that “having to read a footnote resembles having to go downstairs to answer the door while in the midst of making love.”

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The footnote jousting could soon be moot, as the e-book may inadvertently be driving footnotes to extinction. The e-book hasn’t killed the book; instead, it’s killing the “page.” Today’s e-readers scroll text continuously, eliminating the single preformed page, along with any text defined by being on its bottom. A spokesman for the Kindle assured me that it is at the discretion of the publisher how to treat footnotes. Most are demoted to hyperlinked endnotes or, worst of all, unlinked endnotes that require scrolling through the e-reader to access. Few of these will be read, to be sure.

I admit to being somewhat mystified that technological innovation is imperiling footnotes. Computers would seem to solve what I see as the main problem they pose — to wit, edging in the superscript numbers on a typewritten page and measuring just the right amount of space to leave at the bottom. Footnotes really presage hyperlinks, the ultimate interrupter of a stream of thought. (But footnotes are far superior: while hyperlinks can be highly useful, one never finds oneself looking at an error message at the bottom of the page where a footnote used to be.) Even the audio book has solved the problem of how to convey footnotes. Listen to David Foster Wallace reading his essay collection “Consider the Lobster,” with its ubiquitous show-stealing asides: at a certain point, his voice is unnaturally distant, the result of a production trick intended to represent the small type of a footnote. Wallace’s e-book was not immune to de-footnoting, though; all these crucial asides now appear at the end of the book in the Kindle and iPad versions. Even the Kindle edition of Zerby’s history of the footnote is now full of endnotes instead.

Should footnotes fully disappear, I would grieve their loss. I do not find it disagreeable to bend my nose south and find further information where it lands. Surely the purpose of a book is not to present a methodically linear narrative, never wavering from its course, with no superfluous commentary set off by commas. In my mind, footnotes are simply another punctuative style: a subspecies of parenthesis that tells the reader: “I’ve got something else here you might like! (Read it later.)” What better thing? You get to follow the slipstreams in the author’s thinking at your own leisure.

A footnote: I’ve kept this essay annotation-free. But every sentence could have had a footnote, providing a source, further reading, a tangent, an explanation. Is the essay really better without them?