I may have many of my best thoughts in the bath, but I cannot claim to be a second Archimedes: my eureka moment came at the top of a mountain. It was August 1988, the middle Saturday of the Wordsworth Summer Conference. Whereas most conferences offer wall-to-wall papers over two or three days, the annual Wordsworth event in the Lake District moved at a very different pace. It lasted for two full weeks, each morning beginning with a brisk 3-mile walk around the lake, led by the conference organiser, Richard Wordsworth, the poet's great-great grandson. Lectures, paper sessions and seminars were confined to the morning and early-evening slots. The afternoon was to be spent in the open air. Walking in the company of the great Romantic scholar Geoffrey Hartman to the site of the half-built sheepfold described in Wordsworth's poem Michael was as much of an education as an evening in the bar of the Grasmere Red Lion Hotel with Seamus Heaney - poets and poetry readers were as welcome at the conference as distinguished academics and eager students.

On the middle Saturday, there was an all-day hike. As a conference tutor, I was expected to be equally adept with a Variorum Edition of Wordsworth and an Ordnance Survey map. That year I was tasked with leading the A- party around the Coniston Horseshoe. Swirl How, Black Sail Pass, Weatherlam: the very names were enough to lift the spirits as I plotted the route while sitting through the Friday evening's earnest exposition of "Wordsworth's Counter-revolutionary Turn".

The ascent of the Old Man of Coniston goes via derelict slate and copper mines. This prompted a disquisition from an excitable American graduate student on the subject of Wordsworth's failure to see the mountains through the eyes of an industrial labourer. It was the Reagan-Thatcher era and the most influential voices in the literature departments were those of the professors who, in their youth, had been the Class of '68. Surveying the ruins of their own dream of revolution, they seemed to be taking out their disappointment on the Romantic poets. Lucky old Wordsworth and Coleridge, the argument went, they actually saw the French Revolution ("Bliss was it in that dawn to be alive"), but then they turned against it and became political apostates. It was not news to anyone that the later Wordsworth was a conservative figure. The new argument was that his apostasy was inscribed within the very poems that had traditionally been read as the outpourings of his radical youth. The absence of beggars and labourers from the landscape of Tintern Abbey was a sign that the poetry of nature was nothing but a screen for counter-revolutionary sentiment.

The eureka moment came when we reached the summit with the view spread out before us: Morecambe Bay and Blackpool Tower to the south, the Isle of Man to the west, the Cumbrian coast to the north. Three things came together in my mind, and maybe it is that connection of hitherto unconnected ideas that defines a eureka moment.

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The first thought was that the coastline was dominated by the Sellafield nuclear reprocessing facility. I explained to my American graduate student that the name had been changed to Sellafield from Windscale in order to help people forget about a major nuclear accident that had occurred there in the 1950s.

The second thought was that there weren't any sheep trying to eat our packed lunches. I had climbed the Old Man on a family holiday as a boy. As my father was explaining that it was the original for the mountain the Swallows and Amazons children call Kanchenjunga, my mother had been forced to push a sheep away from our ham sandwiches with her stick. Weren't there always interfering sheep on Coniston? "Not this year," replied someone with local knowledge. "They've all been slaughtered because of the caesium levels, the fallout from Chernobyl." Twenty years on, a few Lake District sheep farmers - and more in Wales - are still under post-Chernobyl restrictions.

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The third thought was Brantwood. Someone else had asked me to point out John Ruskin's home overlooking Coniston Water, that day's destination for delegates choosing the coach trip as opposed to the hike. Brantwood, the house where Ruskin spent his tormented later years, raving against the times.

Sellafield, the silence of the sheep, Brantwood. That was it. Eureka: it's the environment, stupid.

Within sight of a capitalist nuclear reprocessing plant, walking on topsoil contaminated by a communist nuclear power station, I remembered all Ruskin's warnings: the argument of Unto This Last, the preface to Munera Pulveris, above all the apocalyptic late lectures on The Storm-Cloud of the Nineteenth Century, in which he propounded the insane argument that the very climate was changing because of human influence, in the form of a toxic combination of carbon emission and moral degeneration. His ravings had begun to look like prophecies. It had taken a hundred years, but the ecological catastrophe he predicted as the consequence of industrialisation and the rage for economic growth was nearly upon us.

Where had Ruskin himself begun from? Surely it was from the Romantic critique of materialism, from not only Thomas Carlyle's Past and Present but also Wordsworth's account in The Excursion of the alienation of industrial wage-labour and the pollution of England's waterways. The political economy of Unto This Last is a wholesale assault on the premise that Karl Marx shared with the capitalism he abhorred: begin, Ruskin said, not with money, labour and production, but with pure air, pure water and pure earth. Humankind can become whole only through connection with the wild flower by the wayside as well as the tended corn. This was the essence of Romanticism; it suggested that we might do well to pause a little longer at the place where our acquaintance with Wordsworth usually begins, with the daffodils.

If, then, Romanticism was a critique of materialism, of capitalism in its high-industrial phase, was it not perverse of my left-leaning American friends to be treating the Romantics as the political bad guys, the avatars of Reaganomics? Then I thought: if we are to use Romanticism to politicise our students, mightn't it be more interesting to use it to show them that trees matter in relation to things that are more far-reaching than landownership? Where the trend in Romantic studies was to attack Wordsworth for not talking about the Enclosure Acts, I wanted to attack his attackers for not talking about deforestation. The Wordsworth of the Guide to the Lakes knew about erosion; he understood that the fragility of ecosystems is a political matter.

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On further investigation, I discovered that Wordsworth's Guide to the Lakes, together with Ruskin's reading of Wordsworth's poetry, were key influences in the history of the British conservation movement. We would not have the National Trust or our national parks without them.

But in my eureka moment I saw that Romanticism did not concern itself only with "shallow green" issues such as national parks and clean air. It also provided a powerful way into "deep green" thinking. Ruskinian political economy asks us to halt in our quest for wealth, for economic growth. Zero growth, Victorian style. It had to be granted that at times the Ruskinian vision smacks of a dark neo-feudalism that troublingly foreshadows some of the more sinister, even quasi-National Socialist aspects of the deep greens ("Back to the land," cried Hitler) - but the very fact that some of the solutions proposed by Ruskin were so draconian might prove a good way of showing up the gravity of the problems that he foresaw. Similarly, recalling that 1798 was the year not only of Lyrical Ballads but also of Thomas Malthus' Essay on the Principle of Population, it occurred to me that to give students parts of Malthus beside Lyrical Ballads would be to encourage them both to historicise and to reflect on pressing contemporary concerns.

I knew that poetry was no longer likely to influence political policy directly, as Wordsworth hoped it would when he sent a copy of Lyrical Ballads to Charles James Fox, but I had faith that as long as it retains a niche in institutes of education, it may still influence mentalities. If the planet is to be saved, I said to myself, we in the super-developed West will have to change our ways; before we can change our ways we will have to change our minds. To change our minds, we need new ways of conceptualising the world. The non-human must be seen as something other than what both Marxism and capitalism see it as, the raw material for production. It must be viewed as Romanticism viewed it, with wonder and reverence, not rapaciousness. (Wordsworth's poem Nutting might thus be read as a miniature allegory of man's rape of nature). The Romantic sublime celebrates the power of the human imagination, but it also powerfully conveys a sense of the insignificance, the smallness, of man. It offers a necessary humbling, a first step towards the knowledge that humankind is not self-sufficient. Like all species, we need our ecosystem; but, unlike other species, we also have the capacity to destroy it - and to destroy every other species' ecosystem.

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Standing on the Old Man of Coniston that August day in 1988, thinking these thoughts in the light of the transnational interconnection between Chernobyl and Cumbria, it struck me that a restatement of the ideals of Romanticism could play some small part in fostering a different way of conceiving of the planet. I had been reading James Lovelock's Gaia, with its startling idea that the whole Earth may be regarded as a single self-regulating organism. But was that such a new idea? What was the Gaia hypothesis but a modern version of the Romantic notion of "the one life that is within us and abroad"?

It was in the era of Romanticism that people began to imagine for the first time the universal rights not only of man and of woman, but also of animals, of nature itself. This, I thought, was a story that might just be worth telling anew as the storm clouds gathered over the late 20th century.

That was the day I began to think about writing the slim polemical book that became Romantic Ecology: Wordsworth and the Environmental Tradition (1991), my first attempt at "green" literary criticism. Not a great work of scholarship, but an intervention of the moment. The model of reading it used was a deliberately transparent one, principally because I wanted to speak not only to my fellow professionals but also to students and amateur readers of Wordsworth (people who discover him in the Lake District, as I did). I took literally what might be described as the ecological didacticism of Wordsworth and Ruskin; I followed Ruskin's injunction to talk about pure air, pure water and pure earth more than money, labour and production.

The ideal I set up was of the small, non-exploitative self-sufficient community: something along the lines of what Samuel Taylor Coleridge and Robert Southey dreamt of establishing on the banks of the Susquehanna River or what Robert Owen initiated at New Lanark or what William Morris aspired to in News from Nowhere and what E.F. Schumacher (another writer who had mattered to me back in the 1970s) imagined in Small Is Beautiful. The predictable result was that the book was much criticised for replicating Wordsworth's own supposed retreat from politics to nature. But it did the necessary work: I described it as a "preliminary step towards a literary ecocriticism" and I'm glad that it's one I took, courtesy of the eureka moment on the Old Man of Coniston.

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