Saunders Mac Lane, an influential mathematician and former Harvard professor who co-founded category theory, died on April 14 from internal bleeding due to constrictive heart disease. He was 95.

Mac Lane was most famous for the ground-breaking paper he co-wrote with Samuel Eilenberg of Columbia in 1945 which introduced category theory, a framework to show how mathematical structures relate to each other. This branch of algebra has since influenced most mathematical fields and also has functions in philosophy and linguistics, but was first dismissed by many practical mathematicians as too abstract to be useful.

Gade University Professor of Mathematics Barry Mazur, a friend of the late Mac Lane, recalled that the paper had at first been rejected from a lower-caliber mathematical journal because the editor thought that it was “more devoid of content” than any other he had read.

“Saunders wrote back and said, ‘That’s the point,’” Mazur said. “And in some ways that’s the genius of it. It’s the barest, most Beckett-like vocabulary that incorporates the theory and nothing else.”

He likened it to a sparse grammar of nouns and verbs and a limited vocabulary that is presented “in such a deft way that it will help you understand any language you wish to understand and any language will fit into it.”

Mac Lane collaborated again with Eilenberg to form a theory of topological space that is called Eilenberg-Mac Lane space.

He also published several college mathematics textbooks with late Harvard professor Garrett Birkhoff, and wrote widely on the history of mathematics.

The Norwich, Conn. local received his bachelor’s degree at Yale in 1930 and a master’s degree from the University of Chicago in 1931, before going on to obtain a doctorate from the University at Gottingen, Germany in 1934.

After brief teaching stints at several colleges, including Harvard, he later returned to the University of Chicago and remained as a professor there for over three decades.

His brother, David Mac Lane, explained that despite his prominence, the famed mathematician was often reluctant to take on administrative posts.

“He was more interested in ideas and creativity than administrative duties,” David Mac Lane said. “Mathematics was the major function of his life....He loved to teach and do research and he enjoyed his work at Harvard very much.”

Mac Lane was president of the Mathematics Association of America from 1951 to 1953. He also served as vice president of the National Academy of Sciences and the American Philosophical Society. He was awarded the country’s highest award for scientific achievement, the National Medal of Science, in 1989, and over his lifetime published 100 papers and authored or co-authored six books.

“[Category theory] set the stage for all modern unifications of mathematics,” Mazur said. “So many theories were begging for it and it is now the lingua franca.”

Mac Lane is survived by his brother, David, his second wife, Osa, two daughters, and a grandson.