Dante had Virgil; Sherlock Holmes had Watson. For over half a century, Noam Chomsky’s closest colleague has been Morris Halle, a revered linguist in his own right, who brought Chomsky to M.I.T., in 1955. At the time, Chomsky was a postdoc at Harvard, brilliant but rarely gentle; no one else, it seems, had the nerve to hire him. “When I met Morris,” Chomsky wrote to me in an e-mail, “what struck me at once was [Morris’s] uncanny ability to see the right answer even if he didn’t have the arguments—and I often have found myself scurrying to try to discover the arguments.”

Together, the two men wrote one of the most important books in the history of linguistics, “The Sound Pattern of English.” Published in 1968, the book did for phonology (the study of the sounds of words) what Chomsky had done for the study of syntax: transformed it, formalized it, and to turned it from a mere collection of facts into a science. Today, the book is known widely by its acronym (SPE), and no text in the field is more central; even those who might disagree with the book refer to it regularly. At the heart of SPE is the claim that the sounds of individual words were derived step by step, through a sequence of rules that come from deep underlying forms, a claim that is parallel to the one that Chomsky was making about syntax.

A few years later, some of Halle’s eminent former students gathered to write a Festschrift honoring Halle’s broad influence in the field of phonology. Festschrifts are not uncommon—many great scientists have been celebrated in this way—but they tend to be a late-career honor. Halle’s Festschrift was published in 1973, when he was only fifty. He had accomplished a lot, but he didn’t slow down; instead, he set his sight on new problems.

By the late eighties, when I arrived at M.I.T. as a graduate student, Halle was pushing seventy years old. He was hard at work on a new theory, called Distributed Morphology, which was aimed at understanding what governs the structures and the endings of words. We can see a hint of this in English, in the rules that dictate morphemes (the smallest grammatical elements, such as verb and noun endings, as well as so-called derivational suffixes, like “-ian,” “-ism,” and “-ity”), which explain why, for example, “Mendelismian” sounds awkward. But, as anyone who has studied Latin or, for that matter, the Native American language of Potawatomi, knows, English is relatively simple when it comes to morphology; in many other languages, the structure of words is vastly more complex. There’s still no consensus, but the theory that Halle helped develop, together with Alec Marantz, has been a major contender for the past two decades.

Today, Halle, who gave a major lecture in January, is being honored with his third Festschrift, “Distributed Morphology Today: Morphemes for Morris Halle,” this one in celebration of the profound influence he has had on linguistics in the past thirty years. He is also, as it happens, turning ninety.

Halle grew up in Latvia, and emigrated to the U.S. in 1940, after Germany invaded Poland. Not long afterward, he was drafted into the U.S. Army, and he served in the Second World War. He then he did a dissertation at Harvard with one of the great linguists of the previous generation, Roman Jakobson.

In the course of his time at M.I.T., Halle has been at least as responsible for building the university’s celebrated linguistics department as Chomsky has. Barbara Partee, one of the authorities on the branch of linguistics known as formal semantics, and the first woman to go through the program, in the early nineteen-sixties, fondly remembers Halle treating her like a “princess.” He took great pride in her presence at the school, a decade before the women’s-liberation movement hit its stride. “If graduate students came because of Chomsky’s name, they thrived in part because Morris was so committed to graduate education,” she recalled. “He advised us on everything, from courses to take to negotiating our first job.”

Halle was a generous mentor, but he was also exacting; “very stern,” in the words of his former student Donca Steriade, who is now one of the world’s leading phonologists. Halle’s sole function on my own thesis committee, which he performed with zeal and good humor, was to cast doubt on virtually every assumption I made. We never reached a consensus, but the work was stronger for his skepticism.

Students who worked closely with Halle recall that he was never satisfied with an idea if it failed to take into account the full complexity of human language. It’s easy to build a linguistic theory about a single, isolated phenomenon, but such theories often fall apart if you take a broader view, looking at many languages in many different systems; Morris never let his students forget that.

Steven Pinker still remembers his first conversation with Halle. Pinker was a postdoc (working with another linguist at M.I.T.), and Halle “explained that the characters of cursive handwriting had a grammar of elementary strokes … More than any linguist I know, Morris has the gift of seeing the simple combinatorial structure underlying a complicated set of phenomena. He’s the Mendele’ev of language—show him one hundred and three elements, he’ll see the rows and columns of the periodic table; show him a quadratic equation, he’ll factor it. This led to his triumph, [SPE] and much of the rest of generative phonology… the elegance, beauty, harmony, and underlying simplicity of sound structure.”

In the mid-nineteen-nineties, Halle’s influence in the field of phonology dropped precipitously. The thesis laid out in SPE had been challenged by something called the Optimality Theory. For a while, it seemed that only Halle and a few of his most loyal students, like Bill Idsardi, still believed the most basic tenet of Chomsky and Halle’s theory, that the pronunciations of words were derived by a sequence of steps (in serial) rather than all at once (in parallel).

But, recently, there have been signs that some of Chomsky and Halle’s ideas may be coming back into favor. Most notably, one of Optimality’s leading advocates, John McCarthy (a linguist at the University of Massachusetts), wrote that Optimality Theory, once the poster child for everything-at-once parallelism, may in fact need to incorporate a bit of Chomsky and Halle’s step-by-step serialism.

No matter the outcome of this particular debate, Halle’s reputation among linguists is secure. He will never be as famous as Chomsky (who has touched popular culture through his politics, as well as through his linguistics), but for generations of students, myself included, he has been a tremendous inspiration. Happy ninetieth, Morris Halle, and we wish you many more.

Photograph by Michael Yoshitaka Erlewine.