Ruth Barcan Marcus GRD ’46, a Yale professor of philosophy and pioneer in the field of quantified modal logic, died Sunday at her home in New Haven. She was 90.

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One of Yale’s first female professors, Marcus helped carve a place for women in academia, and her groundbreaking research in the philosophy of language, ethics, metaphysics and epistemology put her at the forefront of her field. To her students and colleagues, Marcus was an academic visionary and an inspirational mentor.

“She had a kind of personal integrity and intellectual integrity that just shone through,” said Don Garrett GRD ’79, chair of New York University’s department of philosophy and one of Marcus’ former students. “People sometimes found her intellectually intimidating, but anyone who knew her knew that she was a very dear person with a very clear mind — the most logical of philosophers and philosophical of logicians.”

Marcus began her revolutionary work in modal logic during the late 1940s when she developed the Barcan formula, which developed the use of quantifiers in the field. Though Marcus initially came under fire for her radical ideas, she continued her research and eventually drew scholarly recognition and acceptance for her work, her colleagues said.

Born in New York City in 1921, Marcus grew up in the Bronx and went on to attend NYU, where she graduated with a bachelor’s degree in mathematics and philosophy in 1941. Five years later, she earned a doctorate in philosophy from Yale. In 1959, Marcus took her first teaching post as a part-time professor of philosophy at Roosevelt University in Illinois, where she worked until becoming head of the philosophy department at the University of Illinois at Chicago in 1964. Marcus returned to Yale in 1973 as a professor of philosophy and taught in the department until her retirement in 1992.

As a professor, Marcus was known for her toughness, but also for her open and supportive attitude.

While former students said they initially found her intimidating, they added that they quickly recognized Marcus’ warm, generous and funny personality. Former students also described Marcus as an excellent mentor — particularly to women entering academia — and many said she inspired them to pursue careers in philosophy.

“She was a regular mother hen to her students, coddling us along, scolding us [when] necessary,” said Diana Raffman ’75 GRD ’86, a philosophy professor at the University of Toronto and a former student of Marcus’. “The world is a poorer and much smaller place without Ruth in it.”

In addition to holding multiple professorships, Marcus also served as chair of the American Philosophical Association from 1976-’83. While serving in the post, she continued to influence the field of philosophy by increasing transparency in hiring practices at academic institutions across the country. She won numerous accolades for her work, including the Guggenheim Fellowship, Yale’s Wilbur Cross Medal and the American Philosophical Association’s Quinn Prize for service to philosophy.

Walter Sinnott-Armstrong GRD ’82, a philosophy professor at Duke and one of Marcus’ former students, described her as an original thinker whose contributions to logic created a new branch of philosophy.

“She started off way ahead of the philosophical crowd and led them in new directions,” Sinnott-Armstrong said.

Colleagues said Marcus influenced all those around her, even beyond her field. Michael Della Rocca, a Yale professor of philosophy who joined the department the year before Marcus retired, said she always supported her younger colleagues, including him.

Marcus’ talents were not confined to the academic arena — she was also an excellent athlete. As a student at NYU, she was a champion fencer and would have competed at the Olympics had the games not been cancelled because of World War II. Longtime friend and colleague Robert Fogelin GRD ’60 recalled that their squash games were as intense as their intellectual debates.

Friends and family said they remember Marcus as someone who was equally devoted to her personal and professional lives, adding that her strong will and love of learning were her defining traits.

“I think she will always be a model of the philosophical life,” Sinnott-Armstrong said. “She showed how one can be a serious philosopher with very high standards and a compassionate person.”

Marcus is survived by her children, Jim, Peter, Katherine and Libby.

Correction: Feb. 21

A previous version of this article incorrectly referred to the field of quantified modal logic as “quantitative modal logic.” The article also stated that Marcus brought a “quantitative dimension to the field” of modal logic. In fact, she developed the use of quantifiers in the field.