John Rawls, who served as the Conant professor emeritus and was one of the most notable political and moral philosophers of the 20th century, died at his home in Lexington on Sunday. He was 81.

Rawls is credited with reviving the social contract tradition in social and moral philosophy. He devoted his life to the study of the subject of justice, refining and working on his theory for 50 years.

“His achievement in moral and political philosophy is certainly the largest achievement in the English-speaking world since John Stuart Mill’s,” said MIT Professor of Social and Political Philosophy Joshua Cohen.

“He wouldn’t have gone in for rankings, but his work has a place among the greatest tradition of moral and political philosophy and that would include Plato, Aristotle, Rousseau. I expect his work to continue to be studied for the indefinite future,” said Cohen, whose dissertation was advised by Rawls.

Bass Professor of Government Michael J. Sandel recalled his first encounter with Rawls, whom he called “a quiet but towering voice for a more tolerant and generous way of organizing modern democratic societies.”

“In my first year as a young assistant professor at Harvard, the phone in my office rang,” Sandel wrote in an e-mail. “The voice on the other end said, ‘This is John Rawls, R-A-W-L-S.’ It was as if God himself had phoned to invite me to lunch, and spelled his name just in case I didn’t know who he was.”

Rawls’ magnum opus, A Theory of Justice, is regarded as a staple of undergraduate curriculums and is studied by economists, political scientists and legal academics alike.

It argues persuasively for a political philosophy based on equality and individual rights. It describes the reconciliation of liberty and equality, concepts that were viewed as fundamentally at odds for much of the 20th century. Rawls was more egalitarian than any major political party in this country.

“Rawls wrote A Theory of Justice and it was a truly extraordinary achievement both for its intrinsic merit and because it was written against a kind of backdrop of a kind of wasteland in the field,” Cohen said.

Rawls was also an influential teacher for the next two generations of important moral and political philosophers.

“There was nothing sparkling about his teaching. It was not somber, but serious. He taught how to take the history seriously and consider yourself in a kind of conversation with the great thinkers,” Cohen said.

In 1997 Harvard awarded Rawls an honorary Doctor of Laws degree.

In 1999 Rawls was honored in a special ceremony at the White House where President Clinton and First Lady Hillary Rodham Clinton presented him with the National Humanities Medal.

Four weeks ago, Rawls became the second living philosopher to have a Cambridge Companion volume published on him. His wife painted the portrait of him that appears on the cover.

“It is a fitting tribute,” said Samuel R. Freeman, a professor of philosophy and law at the University of Pennsylvania who edited Rawls’ papers and is editing for publication his lectures on political philosophy.