Tom Regan, an American moral philosopher known for his groundbreaking work in the study of animal rights, died of pneumonia Friday, Feb. 17. He was 78.

A professor emeritus at North Carolina State University, Regan was one of the rare philosophers whose work had import and influence outside academia. The former butcher became a vegan and a historic figure in the animal rights movement. Though he wrote many books and papers, his most notable work was "The Case for Animal Rights," published in 1983 near the beginning of the modern animal movement. A monument in the history of animal rights philosophy, it sparked much subsequent debate and was translated into multiple languages.

It is widely recognized as a classic text, but the book has been somewhat overshadowed for popular audiences by its predecessor, "Animal Liberation," the 1975 book by Australian philosopher Peter Singer. Singer was one of the first to eulogize Regan, tweeting Friday afternoon, "I've just heard that sad news that Tom Regan, a philosophical pioneer for animals who I have known since 1973, died this morning."

As peers and collaborators, they sparred intellectually over ideas. Their most central disagreement rested on the nature of "rights," a term frequently taken for granted. And this disagreement set "The Case for Animal Rights" apart from previous work.

In writing a case for animal rights, Regan was not simply adopting the language of other moral and political movements: civil rights, human rights, women's rights. He was distinguishing his view from that of Singer and similar thinkers, known as utilitarians, who reject the notion of "rights" as a conceptual matter.

Utilitarians argue that certain features of the world, such as pain and pleasure, joy and suffering, are good or bad. The right thing to do, utilitarians tell us, is to maximize the amount of good in the world and minimize the amount of bad. They deny that morality is about "rights" — though protecting legal rights to property and freedom, for instance, may be prudent or wise in their view.

Philosophers like Regan have argued that this is wrong. There are certain rights that should not be violated, even if it would make the world as a whole better off. For example, these philosophers might argue that it would be wrong to imprison a wrongly accused woman, even if the suffering imposed on her would be outweighed by the public's desire to see her behind bars. Regan's revolutionary argument was that rights — in particular, the right not to be killed — could be sensibly applied to animals.