Harry G. Frankfurt, a Princeton University philosopher who wrote primarily on Descartes, free will and moral responsibility but found literary fame with his unexpected bestseller “On Bullshit,” a slim treatise on the pervasive, willful and devilish art of avoiding the truth, died Sunday at a care center in Santa Monica, California.

He was 94.

He had several medical issues, including congestive heart failure, his daughter Katherine Frankfurt said.

Frankfurt was thrust into the national spotlight in early 2005 after Princeton University Press republished, in a diminutive 67-page hardback, his brief essay on excrement-level falsehoods that had originally appeared in the academic journal Raritan in 1986.

With his appearances on “The Daily Show With Jon Stewart,” “60 Minutes,” and news programs around the world, Frankfurt’s book achieved a level of publicity rare for an academic publisher and shot to No. 1 on the New York Times bestseller list.

In Frankfurt’s view, the problem at the core of his book was more complicated than simply saying something untrue.

“The essence of bulls— is that the person who produces it doesn’t really care whether what he’s saying is true or false,” he said at the Miami Book Fair. “He’s not engaged in the enterprise of conveying information or in deceiving people. He’s engaged in the enterprise of manipulating opinions, manipulating attitudes and feelings, and he will say whatever he thinks will be effective in that respect, regardless of whether it’s true or false.”

Stewart asked him whether lying or bulls— (a word Comedy Central bleeped out) was more corrosive to society.

“I claim that bulls— is a more insidious threat to society because it undermines respect for the truth and it manifests a lack of concern for the truth,” Frankfurt replied. “It therefore undermines our commitment to the importance of truth.”

Frankfurt wrote the essay as a kind of one-off academic exercise in the 1980s. His primary scholarly interests were “psychology and the philosophy of action,” as he explained during a lecture at the annual meeting of the American Philosophical Association in 2010.

“I don’t remember exactly how I became involved in those areas, but I do recall one pertinent incident,” Frankfurt said.

It was after he finished his book “Descartes, Demons, Dreamers and Madmen: The Defense of Reason in Descartes’s Meditations,” published in 1970.

“I was one day ruminating more or less idly in my office,” Frankfurt said, “about the free will problem; and, in particular, I was turning over in my mind a certain familiar maxim, which was supposed to convey the impossibility of there being such a thing as freedom of the will.”

The maxim, he said, is: “A person may be able to do what he wants, but he cannot want what he wants.”

“It suddenly struck me,” Frankfurt recalled, “as an idea coming entirely out of the blue, that this maxim is manifestly false. It is possible for a person to want what he wants, I thought, just as it is possible that a person does not want to want what in fact he does want.”

He took the idea further, he said, “promulgating the idea that in order to be morally responsible, an agent need not actually have an alternative to acting as he does.”

“What moral responsibility does require, I maintained, is just that the action which the agent performs be something which he not only has a desire to perform,” he continued, “but that it be something which he wants more or less fully and wholeheartedly to perform – that what he does be something, in other words, which he really wants to do.”

David Bernard Stern was born in Langhorne, Pennsylvania, on May 29, 1929, and adopted by Nathan and Bertha Frankfurt. They named him Harry Gordon Frankfurt. His father was an accountant, and his mother taught piano. He never knew his birth parents.

He grew up first in Brooklyn and then in Baltimore. From 1949 to 1954, he studied at Johns Hopkins University, where he earned bachelor’s, master’s and doctorate degrees, all in philosophy.

In addition to Princeton, where he retired as an emeritus professor in 2002, Frankfurt also taught at Ohio State University, the State University of New York-Binghamton, Rockefeller University and Yale University.

Frankfurt’s marriage to Marilyn Rothman ended in divorce. Survivors include Joan Gilbert, his wife of 34 years, of Santa Monica, California; two children from his first marriage, Katherine Frankfurt of Truckee, California, and Jennifer Frankfurt of Los Angeles; and three grandchildren.

His family said he wasn’t big on small talk, but Frankfurt seemed to enjoy – and have fun with – the many interviews he gave.

Asked about his “accidental bestseller” by a New York Times writer, he replied, “What do you mean by ‘accidental’? People didn’t know they were buying it?”

The reporter also asked if he thought the book would have sold as many copies with a “less titillating” title, such as ”On Lying.”

“I think the transgressive aspect of it did have a certain influence on its success,” Frankfurt said. “But the magnitude of the response makes me think that something else was involved. People in this country are starved for the truth.”