Lila Gleitman, whose pioneering work in linguistics and cognitive science expanded our understanding of how language works and how children go about learning it, died on Aug. 8 at a hospital in Philadelphia. She was 91.

Her daughter Claire Gleitman said the cause was a heart attack.

Until the 1970s, most linguists believed that the structure of language existed out in the world, and that the human brain then learned it from infancy. Building on the work of her friend Noam Chomsky, Dr. Gleitman argued the opposite: that the structures, or syntax, of language were hard-wired into the brain from birth, and that children already have a sophisticated grasp of how they work.

“The study of language acquisition, her primary scientific concern, was her field in a special sense: She virtually created the field in its modern form and led in its impressive development ever since,” Dr. Chomsky said in a statement.

Dr. Chomsky, who like Dr. Gleitman received his doctorate from the University of Pennsylvania, devised the theory. But it was Dr. Gleitman who figured out elegant ways to test it in the real world, starting with her own children.

She liked to recount a story about her daughter Claire, then 2 years old. One day when she was driving and Claire was in the car, Dr. Gleitman took a sharp turn and said, “Hold on tight.” Her daughter immediately replied, “Isn’t that tightly?” The utterance showed how even a toddler could understand linguistic nuances, without having been taught them.

Dr. Gleitman called the process “syntactic bootstrapping” — the use of an innate grasp of linguistic structure and its relationship to meaning to figure out new words.

“The kid is really partly discovering what he knows from a complex code in which language is hidden,” she said in a 2013 interview.

Dr. Gleitman often collaborated with her husband, the psychologist Harry Gleitman, or with her graduate students, many of whom went on to become leading linguists themselves.

With Susan Goldin-Meadow, now at the University of Chicago, she showed how even blind children were able to learn “sighted” words like “look” and “see” — not by experiencing them in the world, but by inferring their meaning from their syntactic and semantic contexts. She conducted similar research on deaf students with another former student, Barbara Landau, who teaches at Johns Hopkins University.

“She believed that language learning was not just the accumulation of facts over time, but that it was inherent to who we are as humans,” Dr. Goldin-Meadow said in an interview.

Lila Ruth Lichtenberg was born on Dec. 10, 1929, in the Sheepshead Bay section of Brooklyn. Her father, Ben, was a structural engineer, and her mother, Fanny (Segal) Lichtenberg, was a homemaker.

Lila attended James Madison High School, which educated generations of the borough’s Jews, including Ruth Bader Ginsburg, Senators Chuck Schumer and Bernie Sanders, the economist Gary Becker and Judith Sheindlin, best known as television’s Judge Judy.

She graduated from Antioch College in Ohio with a degree in literature in 1952 and moved to New York, where she took a job as an editorial assistant at the Journal of the American Water Works Association. A few years later she married Eugene Galanter, a professor at the University of Pennsylvania, and moved to Philadelphia. That marriage ended in divorce.

She married Dr. Gleitman, at the time a professor at Swarthmore College, in 1958. He died in 2015. Along with her daughter Claire, she is survived by another daughter, Ellen Luchette, and four grandchildren.

As a faculty wife, Dr. Gleitman could take courses at no cost, and she immersed herself in the classics. But she found her studies boring, except for Greek and Latin.

She entered a doctoral track in linguistics, studying under Zellig Harris, himself a pioneer in the computational study of language, analyzing its deep structures and logic. She also took a job at the Eastern Pennsylvania Psychiatric Institute, where part of her work involved writing entries related to psychology for the next edition of Webster’s Dictionary — including one for a crude term referring to intercourse, which had never before appeared in the book.

“Forever I have taken it as my chief accomplishment in life,” she said in a 2017 interview with Dr. Goldin-Meadow.

Despite becoming one of Dr. Harris’s best students, she was increasingly drawn to the work of one of his leading acolytes, Dr. Chomsky, who was in the process of a fundamental break with his mentor.

Human language wasn’t something that existed separate from the human mind, he argued; rather, it was innate, hard-wired, already there at birth. Dr. Gleitman agreed, and likewise broke with Dr. Harris — a split so acrimonious that he refused to oversee her dissertation.

Nevertheless, Dr. Gleitman received her doctorate in 1967 and began teaching at Swarthmore. In 1972 she returned to the University of Pennsylvania, where she remained until her retirement in 2002.

She did not, however, stop working. In fact, the last two decades of her life were some of her most intellectually fertile.

Working with a colleague, John Trueswell, she studied first how children learn “hard” words — verbs, conceptual nouns — and then turned around and looked at how they learn concrete nouns and other “easy” words, which she argued were not as easy as they might seem.

Dr. Gleitman continued to produce new work even in recent years, after macular degeneration left her nearly blind. Dr. Trueswell said that the last email he received from her arrived the day before she died. It was a short note catching him up on her latest paper — which she had just submitted for publication.