Naess was professor at Oslo from 1939 to 1970, and founder of the ‘Oslo school’. His prolific academic work moved from original involvement with the Vienna circle to an interest in Pyrrhonian scepticism, and an undogmatic and pluralistic epistemology. He was also influential in injecting an empirical and sociological element into semantics, requiring evidence of how language is actually used rather than simple models of how logicians think it is used. He was a celebrated outdoorsman (he made the first ascent of 7,690-metre Tirich Mir, the highest peak in the Hindu Kush, in 1950) and founder of the ‘deep ecology’ movement, enjoining an emphasis on the self-realization and development of all things, based on holistic view of nature deriving from Spinoza’s pantheism. Books include Scepticism (1968) and Ecology, Community and Lifestyle: Outline of an Ecosophy (1989).