Duke University philosophy professor Walter Sinnott-Armstrong has unusual interactions with his students these days.

One contacted him with an excuse for why she was behind in class. She had suffered a personal calamity: Her home in Fiji had been hit by a cyclone. Another claims to be a goat farmer in Afghanistan. And two students — a 12-year-old and her mother — sent the professor a Christmas card from Germany.

They are among the 180,000 students who registered for a class called "Think Again: How to Reason and Argue," co-taught by Sinnott-Armstrong and Ram Neta, a philosophy professor at the University of North Carolina-Chapel Hill.

When it launched in November, "Think Again" was the largest online class offered through a California-based company called Coursera, which has a lineup of more than 200 online courses from 33 partner universities. On Coursera's site, there is a smorgasbord of academic pursuits — quantum physics from the University of Maryland, startup engineering from Stanford or Princeton's "A History of the World since 1300."

All for free, for anyone who wants to jump in to the wild new frontier of global distance education.

"Think Again" is what's known as a massive open online course, an experiment that became a runaway phenomenon in 2012, when top universities rushed to join forces with startup companies promising a higher education revolution.

So far, most offer certificates for courses completed, not grades or college credit, but that appears to be changing. On Feb. 7, the American Council on Education announced it was recommending credit for some online courses.

The whole concept is all at once exhilarating and frightening to U.S. university leaders, who envision both a grand democratization in higher education and scary financial consequences.

Sinnott-Armstrong just sees students, lots of them, learning.

"There are millions of people out there who want better education and can't get it," he said. "This is a way to help them."

He might teach 100 to 200 students a year in his regular classes, or roughly 8,000 over a 40-year span.

There have been 2.5 million separate views of his videos. Each week, the course has up to 10 videos ranging from three minutes to 28 minutes.

Sinnott-Armstrong said the format allows him to present the content in digestible chunks in ways that make sense. He is freed from the constraints of a 50- or 65-minute class period.

It's good for students, too, he said, because if their minds wander, they can replay a video, or if they're not native English speakers, they can stop the video to look up a word in an online dictionary.

Serge Doussantousse, 60, a French-speaking student from Laos, is a researcher who carves out three hours a night for his work. He watches the videos with subtitles. He took "Think Again" because the subject — logic — is important, though it won't impact his career.

"Logic is no fun," he said by email. "I do that to have a better understanding of the world and have a better grip at problems."

What's unclear is how Coursera will figure out the business model for offering free classes. Among the options are charging fees for certificates, matching high-achieving students with employers and licensing courses to universities that couldn't afford to hire top faculty on their own.

Scott Sandell, a Silicon Valley venture capitalist whose firm kicked in $8 million to Coursera, isn't yet worried about making money. The key for now is to develop a good product and a passionate following, he said.

"If you do that well, and you offer something of real value for free, you can end up in a very enviable position in the marketplace," Sandell said in an interview. "You can then begin to monetize the business and find ways to make money. That's especially true if the cost of delivering the service is relatively low, and of course with these Internet-based businesses the costs are low."

Coursera's costs are low because partner universities spend the money to develop the courses. Coursera signs contracts with its partner schools.

Sandell said he isn't troubled that only 14 percent of those who signed up for "Think Again" still are actively taking the course after eight weeks.

"That's more than twice the population of Stanford University in a single course," he said. "That's never happened in history before, and those people are from all over the world. It cost Coursera precisely zero to attract them, not one nickel."

Contact Jane Stancill at jstancill@newsobserver.com.