LAST EVENING, ON the longest day of the year, I took a walk in a meadow near my home.

At the edge of the meadow a path opened into the woods, and I followed it perhaps a hundred yards to the bank of a small stream where I rested on a rock and watched the brook flow. Then I walked back.

Nothing spectacular happened. No large animal jumped out to demonstrate its majesty. The flora was beautiful but unremarkable: buttercups, Queen Anne's lace, daisies, lupins. The sky didn't crackle with summer lightning; the sunset was only streaks of purple, some rosy glow on the underbellies of the clouds. A few mosquitoes made their presence known. It was simply a lovely night.

And simply the sort of scene that we have evolved with for hundreds of thousands of years, that has made us who we are, that we can't be fully human, or at least fully sane, without. The sort of scene whose absence in our lives is now making us slowly crazy. If there is a pertinent modern question, it is "How much is enough?" The consumer societies we have created posit that the only possible answer is "More." And so in pursuit of more we have turned ourselves into tubby folk, raised the temperature of the planet one degree with a further five degrees in prospect, countenanced the ever deeper gulfs between rich and poor, and so on. And in the process made ourselves…happy?

But say you're in a meadow, surrounded by wild flowers. Do you find yourself thinking, "They could do with some more wild flowers over there"? Do you glance up at the mountains on the horizon and think, "Some more mountains would be nice"? Do you lie on the rock by the brook thinking, "This brook needs more rocks"? Does the robin in that tree chide herself for not tripling the size of her nest? I think not. Nature schools us in sufficiency - its aesthetic and its economy demonstrate 'enoughness' at every turn. Time moves circularly through the natural world - next spring there will be wild flowers again. Not more wild flowers: second quarter output for 2005 will show no year-on-year gain. Growth only replaces, since the planet is already accomplishing all the photosynthesis that's possible. It offers the great lesson of being simultaneously abundant and finite.

Interdependent, too. The emergent science of ecology is easily summed up: everything's connected. Field biologists using sensitive detectors have discovered that the needles of trees near Alaskan rivers owe their nitrogen to the carcasses of salmon that die along the banks, the same salmon that feed the bears whose pawing aerates the soil that…

We know now that this is true, but interconnection is anathema to a consumer notion of the world, where each of us is useful precisely to the degree that we consider ourselves the centre of everything. We believe that pleasure comes from being big, outsized, immortal; now our zealots imagine genetically engineering us for greater greatness. But the testimony of the rest of creation is that there's something to be said for fitting in.

And because of that, the natural world offers us a way to think about dying, the chief craziness for the only species that can anticipate its own demise. If one is a small part of something large, if that something goes on forever, and if it is full of beauty and meaning, then dying seems less shocking. Which undermines about half the reason for being a dutiful consumer, for holding ageing forever at bay. Six months from now, on the shortest night of the year, this field will be under two feet of snow. Most of what I can see will be dead or dormant. And six months after that it will be here again as it is tonight.

Advertising, hyperconsumerism, ultra-individualism - these are designed to make you crazy. Nature, like close-knit human community, is designed to help you stay sane. You needn't be in the wilderness to feel in balm: a park, a container garden on the patio, a pet dog, a night sky, a rain storm will do. For free. o

Bill McKibben is a scholar-in-residence at Middlebury College in Vermont and author of Wandering Home.