The commons is an old, simple idea but one that we have never needed so urgently. It’s whatever a community of people shares and manages together. A commons can be anything from a lake that has been fished for centuries to a folk song no one owns to a neighborhood garden to the planet itself. Commoning goes back as long as human history, and it was a basic assumption of the Byzantine emperor Justinian’s legal code and the Magna Carta. It forms the basis for a kind of economics run by neither state nor market but rather by community relationships in which everyone has a personal stake in a shared property or project.

Now, after centuries of being obscured by industrial smoke and no-trespassing signs, people are learning to recognize the commons again. I saw this firsthand at a historic conference last weekend at the Omega Institute — a retreat center in Rhinebeck, N.Y. — called Building the Collaborative Commons.

The first thing one saw upon arriving was a gigantic banner along the parking lot, summarizing the eight principles for managing a commons, culled by Elinor Ostrom, who was awarded the 2009 Nobel Prize in economics. Define clear boundaries, Ostrom’s principles recommended. Ensure that commoners can modify the rules that govern them. Vest responsibility as locally as possible. Up the hill from the parking lot, halls typically used for yoga and spiritual talks welcomed 525 registered participants — many of them leading activists and officials from across the Hudson Valley — to discuss the commons in light of threats facing the region and the rest of the world.

This was, according to the opening speaker, David Bollier, “the first major conference in the United States exploring the commons as such.” He would know. He has been fostering discussions about the commons for decades, and his latest book, “Think Like a Commoner,” is the best introduction to the subject in English. He has watched as people from Europe to South America mobilize around the commons framework while Americans tended to stay on the sidelines. This, however, is starting to change.

Sharing the stage at Omega were speakers such as CNN commentator and Barack Obama’s former green-jobs czar Van Jones, Indian anti-GMO crusader Vandana Shiva, Native American activist and onetime vice presidential candidate Winona LaDuke, futurist and policy adviser Jeremy Rifkin and Bill McKibben, the writer and climate activist behind 350.org and lead organizer of September’s 400,000-strong People’s Climate March in New York City. They had varying degrees of fluency with the idea of the commons, but all were using it in one way or another already — even 14-year-old Xiuhtezcatl Tonatitium. He is a rapper and an environmental activist who has been a plaintiff in a lawsuit against the state of Colorado for failing to protect the environment; the suit relies on the public trust doctrine, an ancient commons-based legal concept.

The commons is a powerful concept for connecting many struggles and issues. Shiva spoke of seeds as a commons, and Rifkin spoke of Net neutrality and the commons of the Internet. Jones spoke of the commoning taking place in the streets of Ferguson, Missouri, in the form of street memorials and protest encampments. In 2009 at the World Social Forum meeting in Brazil, delegates circulated a document that identified the commons as an umbrella for their many struggles over rights to such essentials as land and water. More recently, in the wake of Occupy Wall Street, veterans of the movement turned to the commons as a means of connecting the dots among their disparate grievances. In May I went to Ecuador for a policy summit that proposed the world’s first national transition plan toward a commons-based economy — imagining a society of businesses owned by their workers and customers, open seed libraries for farmers, and indigenous medicines that no drug company can patent.