THE TERM HAS BECOME so widely used that it is in danger of meaning nothing. It has been applied to all manner of activities in an effort to give those activities the gloss of moral imperative, the cachet of environmental enlightenment. “Sustainable” has been used variously to mean “politically feasible,” “economically feasible,” “not part of a pyramid or bubble,” “socially enlightened,” “consistent with neoconservative small-government dogma,” “consistent with liberal principles of justice and fairness,” “morally desirable,” and, at its most diffuse, “sensibly far-sighted.”
 NATURE WILL DECIDE what is sustainable; it always has and always will. The reflexive invocation of the term as cover for all manner of human acts and wants shows that sustainability has gained wide acceptance as a longed-for, if imperfectly understood, state of being.
 AN ACT, PROCESS, OR STATE of affairs can be said to be economically sustainable, ecologically sustainable, or socially sustainable. To these three some would add a fourth: culturally sustainable.
 NATURE IS MALLEABLE and has enormous resilience, a resilience that gives healthy ecosystems a dynamic equilibrium. But the resiliency of nature has limits and to transgress them is to act unsustainably. Thus, the most diffuse usage, “sensibly far-sighted,” is the usage that contains and properly reflects the strict ecological definition of the term: a thing is ecologically sustainable if it doesn’t destroy the environmental preconditions for its own existence.
 ECONOMIC SUSTAINABILITY describes the point at which a less-developed economy no longer needs infusions of capital or aid in order to generate wealth. This definition is misleading: for many of those who use it (including traditional economists and many economic aid agencies), “economic sustainability” means “sustainable within the general industrial program of using fossil fuels to generate wealth and produce economic growth,” a program that is, of course, not sustainable.
 SOCIAL SUSTAINABILITY describes a state in which a society does not contain any dynamics or forces that would pull it apart. Such a society has sufficient cohesion to overcome the animosities that arise from (for instance) differences of race, gender, wealth, ethnicity, political or religious belief; or from differential access to such boons as education, opportunity, or the nonpartisan administration of justice. Social sustainability can be achieved by strengthening social cohesion (war is a favorite device), through indoctrination in an ideology that bridges the disparities that strain that cohesion, or through diminishing the disparities themselves. (Or all three.)
 CULTURAL SUSTAINABILITY asks that we preserve the opportunity for nonmarket or other nonindustrial cultures to maintain themselves and to pass their culture undiminished to their offspring.
 HUMAN CIVILIZATION has been built on the exploitation of the stored solar energy found in four distinct carbon pools: soil, wood, coal, petroleum. The latter two pools represent antique, stored solar energy, and their stock is finite. Since agriculture and forestry exploit current solar income, civilizations built on the first two pools—soil and wood—had the opportunity to be sustainable. Many were not.
 THE 1987 UN BRUNDTLAND REPORT offered one widely accepted definition of what sustainability means: “meet[ing] the needs of the present without compromising the ability of future generations to meet their own needs.” This definition contains within it two key concepts. One is the presumption of a distinction between needs and wants, a distinction that comes into sharp relief when we compare the consumption patterns of people in rich and in poor nations: rich nations satisfy many of their members’ wants—indeed, billions of dollars are spent to stimulate those wants—even as poor nations struggle to satisfy human needs. Two: we face what Brundtland called “limitations imposed by the state of technology and social organization on the environment’s ability to meet present and future needs.”
 AN ECONOMY CAN BE MODELED as an open thermodynamic system, one that exchanges matter and energy across its border (that mostly conceptual, sometimes physical line that separates culture from its home in nature). An economy sucks up valuable low-entropy matter and energy from its environment, uses these to produce products and services, and emits degraded matter and energy back into the environment in the form of a high-entropy wake. (Waste heat. Waste matter. Dissipated and degraded matter: yesterday’s newspaper, last year’s running shoes, last decade’s dilapidated automobile.) An economy has ecological impact on both the uptake and emission side. The laws of thermodynamics dictate that this be so. “You can’t make something from nothing; nor can you make nothing from something,” the law of conservation of matter and energy tells us. With enough energy we could recycle all the matter that enters our economy—even the molecules that wear off the coins in your pocket. But energy is scarce: “You can’t recycle energy,” says the law of entropy. Or, in a colloquial analogy: Accounts must balance and bills must be paid. To operate our economic machine we pay an energy bill; we must ever take in energy anew.
 ESTABLISHING an ecologically sustainable economy requires that humans accept a limit on the amount of scarce low entropy that we take up from the planet (which will also, necessarily, limit the amount of degraded matter and energy that we emit). An effective approach would be to use market mechanisms, such as would occur if we had an economy-wide tax on low-entropy uptake (the extraction of coal and oil, the cutting of lumber). The tax rate could be set to ensure that use doesn’t exceed a limit—the CO2 absorption capacity of the planet, the regenerative ability of forests. Producers and consumers would have freedom under the cap brought about by the tax. With such a tax, the tax on workers’ income could be abandoned. (As the slogan says, we should “tax bads, not goods.” Work is good. Uptake of scarce resources is bad.)
 FOR DECADES environmentalism has been primarily a moral vision, with principles susceptible to being reduced to fundamentalist absolutes. Pollution is wrong; it is profanation. We have no right, environmentalism has said, to cause species extinction, to destroy habitat, to expand the dominion of culture across the face of nature. True enough, and so granted. But even Dick Cheney agreed that environmentalism is essentially, merely, a moral vision. (“Conservation,” he said, on his way to giving oil companies everything they wanted, “may be a personal virtue, but it is not a sufficient basis for a sound, comprehensive energy policy.”) The time has long since passed for the achievement of sustainability to be left to simple moral admonition, to finger-wagging in its various forms. It’s time to use the power of the market—the power of self-interest, regulated and channeled by wise policy—to do good. Environmentalism must become an economic vision.
 ACCEPTING A LIMIT on the economy’s uptake of matter and energy from the planet does not mean that we have to accept that history is over, that civilization will stagnate, or that we cannot make continual improvements to the human condition. A no-growth economy is not a no-development economy; there would still be invention, innovation, even fads and fashions. An economy operating within ecological limits will be in dynamic equilibrium (like nature, its model): just as ecosystems evolve, so would the economy. Quality of life (as it is measured by the Index of Sustainable Economic Welfare, an ecologically minded replacement for GDP) would still improve. If a sustainable economy dedicated to development rather than growth were achieved through market mechanisms, consumers would still reign supreme over economic decision making, free to pursue satisfactions—and fads and fashions—as they choose.
 OUR CHALLENGE is to create something unprecedented in human history: an ecologically sustainable civilization that offers a high standard of living widely shared among its citizens, a civilization that does not maintain itself through more-or-less hidden subsidies from antique solar income, or from the unsustainable exploitation of ecosystems and peoples held in slavery or penury, domestically or in remote regions of the globe. The world has never known such a civilization. Most hunting-and-gathering tribes achieved a sustainable balance with their environments, living off current solar income in many of its forms rather than on the draw-down of irreplaceable stocks, but we can’t say that any of them achieved a high standard of material well-being. Medieval western Europe lived in balance with its soil community, achieving a form of sustainable agriculture that lasted until the invention of coal- and steam-propelled agriculture a few centuries ago, but few of us would trade the comforts and freedoms we enjoy today for life as a serf on a baronial estate, or even for the pre-electricity, pre-petroleum life of a mid-nineteenth-century farmer.
 NO, THERE IS NO PRECEDENT for what we are struggling to create. We have to make it up ourselves.