Wilderness is dead, long live the Wilderness Act. On Sept. 3, 1964, President Lyndon Johnson and the U.S. Congress signed into law the Wilderness Act.

The law was the culmination of a populist movement that began with the founding of Yosemite all the way back in 1890. But the Act was also about a very 20th century American problem: cars. The people who promoted the Wilderness Act year after year before its passage—arch-druids like ecologist Aldo Leopold—worried that roads, and cars bearing tourist hordes threatened to overwhelm the last wild places within the confines of the U.S.

In a very real, legal sense, wilderness equals roadlessness, at least in the U.S.

Here's how the Wilderness Act defines the sometimes problematic term in its name: "A wilderness, in contrast with those areas where man and his own works dominate the landscape, is hereby recognized as an area where the earth and its community of life are untrammeled by man, where man himself is a visitor who does not remain." In fact, some people had to be removed (see Yosemite) to permit any vision of "an enduring resource of wilderness" (in the words of the Act) to become reality.

From the very beginning then, wilderness has been defined by what it isn't: humans and the mechanization we increasingly employ, particularly mechanical transport, whether car, boat or aircraft. The Wilderness Act legislated against the American people themselves, looking to keep us out of certain places, so as to retain the "primeval" character of these places—their flora and fauna—and to reserve them for the enjoyment of fleeting human visitors. The Act represents a gesture of humility and self-restraint.

But most wilderness in the continental U.S. is not untrammeled land. Wilderness areas are often former working landscapes—the Orwellian phrase created by the logging industry to explain away clear cuts—whether they were cleared for logging or farming over the course of the 19th century and early 20th centuries in places like the Adirondacks. The great forest that once covered the eastern U.S. has been re-growing for the last 50 years, even if its primeval quality may be illusory, given the exotic animals and plants that now live there. And, in this era of global warming, even the Artic and other remote spots show signs of human trammeling—whether the leavings are plastic detritus or a changed climate.

So what is still relatively wild in this new epoch, tentatively dubbed the Anthropocene? That's the question facing the Wilderness Act in its next 50 years and one grappled with in a new book of essays called "Keeping the Wild: Against the Domestication of the Earth." "Self-willed" places is the answer most commonly given (though of course nature has no will), whether by British writer and "dark ecologist" Paul Kingsnorth or scholars of old English who note that meaning nestled in the term "wilderness." The 27 authors, including Kingsnorth, collectively deride the hubris and arrogance of modern humanity and call for more of the disciplined humility that the Wilderness Act exemplifies. The alternative is the bleakness of an "existential apartheid" in the words of sociologist Eileen Crist of Virginia Tech that translates into an estrangement from all other species. Or as conservation biologist Michael Soule of the University of California, Santa Cruz puts it: "If you live long enough you will discover that most of the wild places you loved as a young person have been peopled to death."

The Wilderness Society is still out there defending the country's more than 100 million acres of wilderness from the threat of peopling. Most often by finding justification for its continuing use to humans: helping study climate change or filtering clean water for our drinking needs. And, of course, that 109 million acres is less than 1 percent of the total acreage in the U.S.

The question of defending such natural resources quickly devolves into how much people should intervene to maintain wilderness, particularly for iconic animals or favored plants. Should boundaries be shifted to accommodate the movement of species as climate changes? Or does less mean more when managing wilderness?

That's what Roger Kaye, an environmental thinker and wilderness coordinator for the U.S. Fish and Wildlife Service in Alaska, argued in his history of the Arctic National Wildlife Refuge in the International Journal of Wilderness back in 2010, the 50th anniversary of the designation of that refuge. The wildness of wilderness is what needs preserving most now, the plants and animals and even geology of a given place's ability to determine its own fate. Kaye means that an ecosystem should be left to its own natural devices. Plants and animals can take their own course, without any planning, and may change not to anyone's liking. That in itself may be the ultimate resource wilderness provides—something outside the totality of human civilization.

In the Anthropocene, we will have our novel ecosystems that mix old and new plants and animals, and perhaps even missing species resurrected thanks to synthetic biology. We may even intervene purposefully on a planetary scale to stave off the worst warming from ever-rising CO2 levels as we burn more fossil fuels. But the wilderness will remain in some form, going about its own business in its own way.

Wilderness poses this fundamental question at least: what kind of place do we want for our home? Will our terrestrial abode retain an abundance of plants, animals, microbes and fungi like the world Homo sapiens was first born into? Or will the Earth become a vast monoculture, a grim subset of nominally wild species that co-exist in symbiosis with modern human civilization, like rats and seagulls? "Is being an asteroid the great purpose of our species—to steal the lives and homes of millions of species and billions of creatures?" asks political scientist David Johns of Portland State University, in his essay in "Keeping the Wild."

In the end, wilderness is a state of mind. The natural world can only persist now as a deliberate act of human will. That will require firm human purpose as a gesture of humility, yes, but also a form of self-protection. "This is not really an 'environmental problem.' It's a human problem," writes environmental historian Roderick Frazier Nash of the University of California, Santa Barbara. "What needs to be conquered now is not the wilderness, but ourselves."