By Laurence H. Tribe, Oxford University Press, New York, N.Y. 304 pages, $19.95

In Italo Calvino’s 1972 novel “Invisible Cities,” the Venetian traveler Marco Polo tells the emperor Kublai Khan about 55 impossible magical cities, from Zobiede, a city of sleeping men, to Ersilia, a city built on strings, and Argia, a town clogged with earth. Each of these fantastical cities that Polo conjures, however, is both echo and fragment of Polo’s native Venice, only glimpsed through Calvino’s fantastical imagination.

The relation between Venice and Marco Polo’s tales is the closest I can find to the similar uneasy yet productive relationship between the textual Constitution with which lawyers and civics students are familiar and the “invisible constitution” described in Harvard law professor Lawrence Tribe’s new book.

Like “Invisible Cities,” Tribe’s short and accessible (if sometimes dippy) book, “The Invisible Constitution,” is a serenade to an original inspiration – here, the achievement of the U.S. Constitution and the stable political order it has generated. Never perfect, sometimes fraught with great evils – slavery for one – and never finally settled, the Constitution in Tribe’s view is an experiment in self-government animated by principles of equality and human dignity. This experiment, Tribe argues, is not limited to the document’s text. It extends also to principles and practices that have accrued to operationalize the text. There is not, and cannot be, a bright line for Tribe between “the Constitution” as written down and “constitutional law” in practice.

For most lawyers, Professor Tribe is the author of the most respected and authoritative synoptic treatise on the corpus of constitutional law. In May 2005, however, Tribe announced he would not be completing the second volume of his treatise’s third edition. (The first is in print and an essential reference text for scholars and constitutional lawyers). He had come to the “sobering realization that no treatise, in my sense of that term, can be true to this moment in our constitutional history.” In a letter announcing his decision, he explained that he did “not have, nor do I believe I have seen, a vision capacious and convincing enough to propound as an organizing principle for the next phase in the law of our Constitution.”

“The Invisible Constitution” seems to emerge from the impasse animating Tribe’s decision to cease work on his treatise. Tribe has always had a remarkable breadth and generosity of vision; in 1992, he published a piece in the Harvard Law Review drawing on relativity theory to explain the Constitution (a piece, incidentally, that President-elect Barack Obama worked on), and in this book riffs on Kurt Gödel’s incompleteness theorem to explain why it is impossible to understand the written constitution by its text alone. Like Marco Polo, Tribe perhaps contains too many stories, too many perspectives to be contained. Where a treatise might founder on such richness of vision, an essay can dip and soar gracefully between ideas without committing to any one.

Tribe’s new book is a populist exploration of one of the most important trends in the study of constitutional law: the identification, study, and celebration of the Constitution outside its text and outside the courts.

Despite the triumphant rise of “originalism” and “textualism” as interpretive philosophies, scholars in the last decade or so have been possessed of an increasing sense that the written Constitution does not exhaust the legal materials that make up our founding corpus and that courts are not the only interpreters of constitutional rules. Just as the British constitutional scholar Walter Bagehot distinguished between the dignified and the efficient constitutions, so too, American histories are starting to see invisible wheels in and around the textual cogs.

In the most elegant presentation and synthesis of the first point, published last year in the Yale Law Journal, Duke law professor Ernest Young explored how landmark statutes such as the Administrative Procedures Act or the post-Civil War civil rights statutes play a constitutional function. Without them, Young explained, the constitutional order would not simply be impoverished, it would be but nonfunctional in critical ways.

Just as our sense of what is constitutional has expanded, so too has our sense of who interprets the Constitution, that is, of who practices constitutionalism. Hence, political scientist Keith Whittington has argued that the political branches engage in “constitutional constructions” that bridge the gaps in the original text. Legal scholars such as Larry Kramer and Mark Tushnet, by contrast, have sought to reclaim an ideal of popular constitutionalism, wherein popular movements lacking a formal, institutional home have a role to play in developing constitutional norms.

One might understand Tribe’s book as an exploration of the constitution outside the written constitution written in order to encourage popular constitutionalism. From his beginning anecdote about teaching fifth-graders about the Constitution, Tribe makes clear the virtue he finds in self-government. Indeed, this becomes an ideal animating the varied explorations of the book, which is the latest installment in an excellent series aimed at a general lay readership.

In a rapid-fire series of short, sometimes breathless, chapters, Tribe explains and tests the many reasons why the Constitution cannot be understood by the text alone. Along the way, Tribe introduces neophyte readers to some of the great debates within constitutional law from the rise and demise of “economic due process” at the beginning of the 20th century, to the still-smoldering conflict around Roe v. Wade’s legitimacy.

The terminology Tribe uses to map his chosen terrain is unusual. Few scholars have considered previously either the “geological” or the “gyroscopic” constitutions that Tribe describes among the six pictures he draws – literally, in a sheaf of hand-drawn and colored-glossy inserts found late in the book – of his constitution beyond the document’s text. However unusual their presentation, though, Tribe’s ideas correspond to familiar intuitions about constitutional law. (“Geological” understandings of the Constitution reflect efforts to maintain democratic decision-making processes that are open to all. The “gyroscopic” principle is simply the notion that different constitutional principles accommodate conflicting forces in healthy tension). I am not certain the six ideas add up to something greater than their parts, or that the abstruse alliterative terminology Tribe selects is a helpful way of introducing his ideas.

“The Invisible Constitution” is never less than entertaining and informative, even as it shows that professor Tribe has more than enough visions “capacious and convincing enough” to propel the next elaboration of constitutional law.

Aziz Huq is associate counsel at the Brennan Center for Justice at New York University School of Law.