Truly transforming ideas are instantly transferable. They belong to no one. They do not support dogma or ideology, they mobilise action and choice and harness them to a broader, shared purpose. The idea that the earth needed friends was just such a transforming idea, compelling in its simplicity. No one who ever caught and held it for a moment was ever quite the same again. I certainly was not.
Approaching the end of the 20th century, it seems superfluous to argue that the earth needs friends. The reasons why are appallingly obvious. In the deserted lands around Chernobyl; the warning in too many cities on too many days that the air is not fit to breathe; in the growing legion of shore-bound fishermen who left themselves too few fish to catch. Twenty five years ago, it was a different matter. Then it was a novel and much misunderstood idea.
It was also a very different world. There were more than a billion fewer people on the planet. Public awareness then about the environment was low. There were only a few tiny and largely ignored environmental organisations. Such public discussion of the environment as was intermittently reported took place among a few renegade scientists and those Tony Benn later called "the brown bread and sandals brigade". The corporate world was a vast pool of ignorance and indifference to the environment.
Today, there is a huge amount of national and international environmental legislation, much of it, sadly, more honoured in the breach than the observance. Throughout the world the many departments of the environment grow steadily in influence over other departments of government. There are thousands of environmental organisations with tens of millions of members and loud and increasingly listened-to voices - ask Shell. Public awareness about the environment has never been higher and environmentalists now wear suits, a phenomenon that was a never-ending source of wonder to the then Permanent Secretary when I joined the Department of the Environment as a special adviser in 1991.
Of course, Friends of the Earth did not accomplish all this on its own. It belongs to a tradition that has deep roots in Britain's intensely democratic culture. There have been other activists: Greenpeace for one, the organisation Sustainable Development for another, that have also mobilised and focused change. Perhaps most important has been the influence of the unseen actors - throughout the world young daughters, whose influence on their businessmen fathers has proved to be immense, have done as much as anyone to roll back the indifference and ignorance of the corporate sector.
This is fortunate since the easy politics of the environment are now over and the hard politics about to begin. The easy politics have been largely about tackling threats that were readily apparent to most people. The cost of remedies, despite loud complaints from a few, has been small. Politically, the number of winners has far outnumbered the losers, making action possible if not always easy.
The hard politics will be very different. They are about securing the productivity of the ecological base on which a surprisingly large proportion of our economic well-being still depends. The reasons for acting are often not at all obvious to most people and the cost of adjustment will be real and large and only exceeded by the cost of failing to adjust. In the short term, the number of losers will be as great, or greater than, the number of winners.
The brutal truth about the politics of the environment is that all the successes of the past quarter century have been no more than tactical victories in the long retreat for the environment in the face of inexorable pressure from a rising population with rising income that it spends unwisely.
The agenda of the easy politics is very familiar: air and water quality; waste from contaminated land; endangered species, chemicals and radioactivity. The successes in tackling these issues have been real, especially in the West, and they will continue. The new environmental agenda is unfamiliar: food security; fish stocks; water availability; forests; climate change, transport policy. These are the issues that will have to be tackled successfully in the next 25 years if our grandchildren are to inherit prospects as good as we did.
Just how difficult this will be is well illustrated by some recent events. The recent conversion of the US Secretary of State, Warren Christopher, to the environmental cause has been driven in a large part by his enforced understanding that there can be no lasting peace in the Middle East without an agreement among the warring parties about how to distribute the region's scarce water resources in the face of its burgeoning population.
This is a problem that will become more acute if climate change has the anticipated effect of making dry areas drier. In the past 12 months there have been more international conflicts over access to fish stocks than over any other matter. A pressure that last year brought Nato navies within a hair's breadth of shooting at each other. World grain prices have rocketed this year, undermining the small gains made recently in reducing the environmental impact of agriculture.
The choices that the hard politics of the environment force on us all will be no easier for the environmental organisations. They will require a more sophisticated political approach than that on show during the dramatic simplicities of Greenpeace's campaign against Brent Spar. Their age of innocence is over as their agonising about application of VAT to domestic energy use so clearly demonstrated.
There are many more such dilemmas ahead. How, for instance, can we reconcile the popular support for cultural diversity relating to protection of the rainforests and their indigenous peoples, with the creation of global values and the education of women or management of population size?
There is no more dangerous drug for those involved in public debate than being right. Success all to easily breeds self-righteousness. Friends of the Earth has been right about the folly of relying on road building to solve our transport problems. It was also right to warn that nuclear reactors burnt money better than they burnt uranium. But it does not have a monopoly on wisdom.
As we face the more difficult choices of the future it will need to learn to co-operate as well as confront, to solve as well as to shout, while losing none of its dynamic aggression. The earth that needed its friends 25 years ago needs them even more today, but they need to be smarter, more confident, more mature and, above all, more committed than ever.
The writer, a former director of Friends of the Earth, is now a special adviser to John Gummer.