Næss averred that while western environmental groups of the early post-war period had raised public awareness of the environmental issues of the time, they had largely failed to have insight into and address what he argued were the underlying cultural and philosophical background to these problems. Naess believed that the environmental crisis of the twentieth century had arisen due to certain unspoken philosophical presuppositions and attitudes within modern western developed societies which remained unacknowledged.
He thereby distinguished between what he called deep and shallow ecological thinking. In contrast to the prevailing utilitarian pragmatism of western businesses and governments, he advocated that a true understanding of nature would give rise to a point of view that appreciates the value of biological diversity, understanding that each living thing is dependent on the existence of other creatures in the complex web of interrelationships that is the natural world.
Næss was born in Slemdal, Oslo, Norway, the son of Christine (Dekke) and Ragnar Eide Næss. In 1939, Næss was the youngest person to be appointed full professor at the University of Oslo and the only professor of philosophy in the country at the time. He was a noted mountaineer, who in 1950 led the expedition that made the first ascent of Tirich Mir (7,708 m). The Tvergastein hut in the Hallingskarvet massif played an important role in Ecosophy T, as "T" is said to represent his mountain hut Tvergastein. In 1958, he founded the interdisciplinary journal of philosophy Inquiry.
In 1970, together with a large number of protesters, he chained himself to rocks in front of Mardalsfossen, a waterfall in a Norwegian fjord, and refused to descend until plans to build a dam were dropped. Though the demonstrators were carried away by police and the dam was eventually built, the demonstration launched a more activist phase of Norwegian environmentalism.
In 1996, he won the Swedish Academy Nordic Prize, known as the 'little Nobel'. In 2005 he was decorated as a Commander with Star of the Royal Norwegian Order of St. Olav for socially useful work. Næss was a minor political candidate for the Norwegian Green Party in 2005.