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Arne Naess: Philosopher who invented the concept of 'deep ecology'

I have learned as much from my rats as I have learned from Plato," Arne Naess informed a startled Karl Popper. Naess was the highly influential Norwegian philosopher whose ideas about ecology and humans' relationship with the environment have informed and enriched many of today's green activists and movements. His key notion of "deep ecology" is the idea that all of nature matters and deserves equal consideration, not just those parts that impinge upon humans. Among his 30 books, both technical and popular, and hundreds of papers, were such bestsellers as Life's Philosophy: reason and feeling in a deeper world (2002) which made him the man whom Norwegian teen-agers most wanted to meet. Although the environmentalist Bill McKibben called this good-humoured, ever-welcoming creator of "ecosophy" a "universal great-grandfather", Naess shied away from the idea of disciples.

Naess was born into a wealthy family outside Oslo in 1912; his father, Ragnar, died a year later. His mother, Christine, had sublimated dramatic aspirations into bringing up six children, of whom two died young, and widowhood exacerbated her over-attentiveness. Naess resisted this, becoming somewhat emotionally detached, and preferred his governess, Mina. Childhood observation of marine life-forms offered refuge, as did mountains, especially the Hallingskarvet range near Oslo, where he would later spend large portions of his adult life. At 15 he went climbing there alone and, on encountering a hermit, spent a week with him: porridge and dried bread were leavened by the fellow's violin.

Naess was adept in languages, including Sanskrit, and read Spinoza in Latin, where he warmed to the belief that "human nature is such that the sight of others' happiness releases happiness in ourselves". Spinoza guided him forever. From Oslo University, where he studied philosophy in the early Thirties, he went to Paris and then Vienna, where he spent just over a year. Here his first book – Truth as conceived by those who are not professional philosophers, based on questionnaires given to ordinary people – was published in 1938. He underwent analysis with a disciple of Freud six days a week for 14 months, after a fellow student inferred that his enjoyment of organ concerts denoted a psychological lack. He was also given the opportunity to observe mental patients – disguised in a white coat – before returning to Norway.

There, in the mid-1930s, he began retreats to a mountain hut at Trevgastein [Crossed Stones]. These did not daunt his first wife, Else, whom he had married in 1937, and with whom he had two sons. Theirs was the companionable union of friends (they had known each other since the age of seven) and Naess had various lovers, too.

In 1938 they went to Berkeley, where Naess studied behavioural science under E.C. Tolman. He digressed into observing rats' struggles within a maze – and, what's more, the scientists' behaviour, something dismissed by his fellow philosopher Popper as "zoology".

At 27 ("emotionally in my teens", as he later described himself), he became Norway's only Professor of Philosophy. With the German invasion in 1940, thought met action. Steeped in Gandhi, Naess advocated non-violent resistance rather than outright sabotage. (Indeed, the Geneva Convention decreed that any fighters had to be in uniform: the casually clothed could be summarily executed.) To begin with, the university remained open, but by 1943 things had changed. Naess, by now a discreet resistance member, received information that many students would be sent to concentration camps. He prevented the despatch of some; others were caught while debating whether the warning was a hoax. In post-war years, as well as working on democracy for Unesco in Paris, he supervised investigations into those who had never returned. He did not disclose what tactics the Norwegian resistance had used, lest they be needed after a Soviet invasion.

Adroit organisation meant that he spent much of each week in the Norwegian mountains, and climbed others around the world; he was part of the first expedition to climb Tirich Mir, in Pakistan, in 1950. By now, he and Else had separated and he married Siri, with whom he had a daughter. Visiting professorships included strife-ridden, pot-heavy Berkeley in 1968. He was there persuaded to try LSD, and later told the story of a mother telephoning shortly afterwards to explain that a son's essay would be late because he had been bitten by an animal in Mexico; Naess's reply was even more surreal. Always ready with humorous anecdotes, he once recalled Nato's Halvard Lange stating that he was essentially a pacificist; Lange's wife whispered to Naess, "then I am a virgin".

Belated reading of Rachel Carson's Silent Spring (1962) led Naess, back in Oslo, to resign from his philosophical post and pursue ecological concerns. Early protests led to a key role in Greenpeace. In 1972, in Budapest, he first spoke of "deep ecology"; that is, a whole way of self-sufficient being, amid the world's creatures and plants, rather than simply forsaking plastic carrier-bags.

He was not, however, dogmatic, and, indeed, benefited from his rich siblings' subsidy of foreign explorations, including the Hong Kong visit on which he met his third wife, Kit-Fai. (His nephew, also called Arne Naess, was a shipping tycoon who was married for some years to Diana Ross, and died in a mountaineering accident in 2004.)

Naess's Ecology, community and lifestyle (1976, translated 1989) anticipated many green concerns and tactics, such as "fifth columnists" within established political parties, while continually emphasising life's joy (as does a volume of conversations with David Rothenberg). Central from the start was that "humankind is the first species on earth with the intellectual capacity to limit its numbers consciously and live in an enduring, dynamic equilibrium with other forms of life". So integrated was his own life that he last took a holiday in his teens.

Christopher Hawtree

Arne Dekke Eide Naess, philosopher and ecologist: born Slemdal, Norway 27 January 1912; married three times (two sons, one daughter); died Oslo 12 January 2002.