By the late 1960s the environmental movement had spread but its philosophical ground remained largely instrumental: pollution was a problem because it harmed human health, deforestation because it exhausted resources. In a short paper of 1973 the Norwegian philosopher Arne Næss called that stance 'shallow ecology' and set another beside it.

Deep ecology's core claim is that the value of living beings and ecosystems is independent of their usefulness to humans. Næss gives what he insists is an ontological rather than a mystical reason: as the self deepens it takes in the web of relations it lives within, and for someone who sees the river, the forest and other species as part of an enlarged self, protecting nature is not sacrifice but self-preservation. He calls this the *ecological self*. The approach's eight-point platform was formulated with George Sessions in 1984; the point calling for a reduction in human population and intervention is the sharpest part of the argument.

Criticism came from two directions. Social ecology (Murray Bookchin) located the source of the problem not in the human-nature relation but in relations of domination among humans, and charged the approach with political blindness; ecofeminism and the environmental justice movement argued that positing 'humanity' as a single subject distributes responsibility evenly and thereby hides the actual polluters. Næss's own life reads like a thesis: he was a mountaineer who made one of Norway's hardest climbs and lived for years in a hut he built on Hallingskarvet. Today legal developments such as nature gaining juridical personality — the Whanganui River in New Zealand in 2017 — have carried deep ecology's question into the courtroom.