Ruth Barcan Marcus was born in New York City on August 2, 1921, and grew up in the Bronx. She was the third daughter of first-generation immigrants of Eastern-European origin. Her mother was a homemaker; her father, a printer and contributing writer at the Jewish Daily Forward, and an active member of the Socialist Party. Her father died when she was eight years old, and her mother supported the family by taking in boarders.

She attended public school, including a selective junior high for intellectually precocious students, where one of her classmates was the future writer Grace Paley. She went on to receive a B.A. in mathematics and philosophy from New York University in 1941 and a Ph.D. in philosophy from Yale in 1946. In addition to her academic extracurricular achievements (she edited a literary magazine and presided over the mathematics and the philosophical societies), Marcus was a champion fencer. Indeed, while she was a graduate student at Yale it was that sport that drew her to Jules Alexander Marcus, a physicist and also a medaled foil fencer, whom she married in 1942. The couple had four children and later divorced.

Marcus spent the early years of her career in various postdoctoral fellowships and visiting positions, including a Guggenheim Fellowship (1953–1954). She taught at Roosevelt University from 1959 to 1963, and in 1964 she was appointed head of the philosophy department at the University of Illinois at Chicago. She then served for three years as a professor at Northwestern University (1970–1973) and, in 1973, was appointed professor of philosophy at Yale. She retired as the Reuben Post Halleck Professor in 1992.

A pioneer in the field of logic and metaphysics who gave her name to a groundbreaking formula of symbolic logic and was a key figure in philosophical debates, she first came to prominence in the world of metaphysics in March 1946 with the publication of an article in The Journal of Symbolic Logic in which she proposed a formula for positing a connection between possibility and identity.

Shelly Kagan, a colleague in the Yale philosophy department, says the Barcan axiom, as it became known, “proposes certain connections between certain claims about what might (or might not) be possible, with other claims about what there might (or might not) be.”

Oxford professor of logic Timothy Williamson sums up the Barcan formula this way: “If there can be something that has a certain property, then there is something that can have that property.”

“This formula gets at certain central issues about what metaphysically grounds possibility,” notes Kagan. “More generally, the ‘quantified modal logic’ which Ruth helped work out is central to a great deal of contemporary philosophy.”

Following her noteworthy debut on the academic stage, Marcus went on to write seminal papers on essentialism, epistemology, belief, language, and ethics. Her major contribution in the last category was her work on moral dilemmas — i.e., cases in which one has an obligation to do x and an obligation to do y, even though it is not possible to do both x and y. She rejected the then-prevailing view that any moral code that does not rule out the possibility of moral dilemmas is thereby inconsistent. A much-acclaimed collection of her essays, “Modalities,” was published in 1993.

Yale professor of philosophy Michael Della Rocca notes: “Her decisive work not only founded an entirely new domain of logic (‘quantified modal logic’) but it also made possible a new understanding of the ways in which proper names and similar linguistic devices play a role in our thought about the world. And those implications of her contributions show no sign of being exhausted.”

She was a fellow of the American Academy of Arts and Sciences, and served as president of the Association for Symbolic Logic (1983–1986) and chair of the board of the American Philosophical Association (1977–1983). In 1995, she was awarded an honorary doctorate of humane letters by the University of Illinois.

“Not afraid to make enemies and blessed with many loyal friends, Ruth was unrelenting and consistent in upholding the highest standards for rigor and clarity in philosophy and in academia more generally,” said Della Rocca, echoing the sentiments expressed by many friends, former students and colleagues responding to the news of her death.