Our lives are governed by unlikely constructions of jargon, self-contradicting oddities that sometimes come crashing to the ground (mutually assured destruction, Progressive Conservative), or stay perilously aloft despite their contradictions (constitutional monarchy) or give rise to new impossibilities (pre-emptive self-defence).

This fall, Canadian debate is full of the clangorous tones of another such verbal collision: sustainable development. We come to it by way of our painful struggle over the Kyoto Protocol, and through Jean Chrétien's recent promise to double our foreign aid for "sustainable" projects.

In other places, it's been a prominent concern for 15 years. If you spend even a short time in the hallways of the United Nations, you will hear it uttered endlessly. Dropping the phrase "sustainable development" in UN circles is the equivalent of mentioning Jesus at a Republican Party gathering.

But just when we're first applying it at home, "sustainable development" is beginning to fall apart. For such a successful phrase, it has always eluded precise definition. Grade-school children now learn the 1987 explanation that brought it into the mainstream, by Norwegian prime minister Gro Harlem Brundtland: "The ability of humanity to ensure that it meets the needs of the present without compromising the ability of future generations to meet their own needs."

This is fine and worthy, and about as applicable as "buy low, sell high." So environmentalists, governments and agencies have published libraries full of more precise formulations: As tabulated at the Massachusetts Institute of Technology, the phrase now has as many as 57 competing definitions.

For a while, this vague term seemed to hold together simply because of its success. The "sustainable-development project" was the upbeat global story of the 1990s: the hand-pumped well in Sudan, the tiny loans to working women in India. Leftists liked it because it didn't involve big corporations, and rightists liked it because it didn't involve a lot of government spending (even Margaret Thatcher gave SD a heartfelt endorsement).

And most impartial observers agree that the switch from huge, corrupt projects to "sustainable" aid is among the reasons why living conditions for the world's poor have improved in the past decade.

But now its opponents are rallying. The first group I'll call the anti-sustainablists. They seem to be headquartered in England, their de facto leader being Julian Morris of the International Policy Network. Mr. Morris and 16 of his economist friends have caused a stir over there by publishing a collection of essays arguing that the "sustainable" imperative has got in the way of economic growth.

These essays tend to have an astringent, tinny flavour, written from a strict libertarian perspective: All foreign aid is by definition bad, growth can be achieved only through open markets, and so on. Still, beneath the ideological barbed wire lies a point that cuts to the heart of aid politics: The countries that are the cleanest and have the most protected land are those that are the richest, such as Canada.

"The rich world in particular has adopted, for the most part, institutions and policies that are sustainable," Mr. Morris writes, echoing Indira Gandhi's famous line about poverty being the greatest polluter.

This point is not easily dismissed: Should we rush to make the world wealthier first, so that cleanliness will then take care of itself? If so, then keep your sustainable out of my development.

From the other side of the spectrum come the anti-developmentists. Their vanguard is led by the hard ecologists, who believe that sustainable development is wrong because it involves growth. To enviro-economists Herman Daly and Kenneth Townsend, SD is "a bad oxymoron -- self-contradictory as prose, and unevocative as poetry." They argue that the earth will benefit from not just an end to human growth, but a reversal.

This critique will always hold limited appeal, since its solution involves banishing us all to the tyranny of nature.

Recently, though, a more subtle critique has appeared. In his book Myth and Reality in the Rain Forest, New York anthropologist John Oates describes how the stolid old practices of wildlife conservation in West Africa got subsumed into the novelty of community-based economic development aid, which ended up destroying wildlife sanctuaries. In the name of conservation, he concludes, keep your development out of my sustainable.

The two camps almost certainly aren't more than marginally aware of each other's existence. They probably wouldn't want to sit at the same table, or even eat the same foods, but together they point to a central weakness in sustainable development.

People in the ideological middle have begun to propose that "sustainable" and "development" be turned into separate missions. Gregg Easterbrook, a U.S. renegade environmentalist, has come out in favour of vastly increased foreign-aid spending and Kyoto programs -- but without sustainable development.

"Sustainable development is one of the vaguest phrases in human history," he told me the other day. "The organizations that advocate it would never dream of living under the conditions that they want the impoverished of the developing world to live under."

After all, we all want to be rich, and we all want to be clean -- but not necessarily at the same time.