It formed part of a broader personal philosophy that Mr. Naess called ecosophy T, “a philosophy of ecological harmony or equilibrium” that human beings can comprehend by expanding their narrow concept of self to embrace the entire planetary ecosystem. The term fused “ecological” and “philosophy.” The T stood for Tvergastein, his name for the mountain cabin he built in 1937 in southern Norway, where he often wrote.

Image Arne Dekke Eide Naess Credit... Erlend Aas/Scanpix Norway, 2004

Arne Dekke Eide Naess was born in Slemdal, near Oslo, in 1912. His older brother was the shipping tycoon Erling Naess, who died in 1993. After earning a degree from the University of Oslo in 1933 Arne Naess continued his education in Paris and in Vienna, where he became part of the Vienna Circle, a philosophical school dedicated to empiricism and logical analysis. In the belief that philosophers should be self-aware, he also underwent psychoanalysis.

After completing “Knowledge and Scientific Behavior,” his dissertation, in German, he was given a teaching position at the University of Oslo, where, as Norway’s only professor of philosophy until 1954, he was the animating figure in the Oslo School. Working in teams, the Oslo School’s adherents used questionnaires to investigate the meanings that ordinary people assigned to terms like “truth,” “free enterprise” and “democracy.” In 1958 he founded the journal Inquiry.

Over his career, Mr. Naess progressed from a radical empiricism to pluralism and skepticism. In his many publications, he took on a wide variety of philosophical problems. Harold Glasser, the editor of “The Selected Works of Arne Naess” (2005), has called him “the philosophical equivalent of a hunter-gatherer.” He was interested in language, meaning and communication, a subject he wrote about in “Interpretation and Preciseness” (1953) and “Communication and Argument” (1966), and in the relationship between reason and feeling. He also wrote books on two thinkers central to his worldview, Spinoza and Gandhi.

In 1969 Mr. Naess left the university to develop his ecological ideas, which, he believed, demanded political action. With other environmentalists, he chained himself to rocks in front of the Mardal waterfall, successfully pressing the Norwegian government to abandon plans for a dam on the fjord that feeds the falls. He also wrote extensively on the ethics of mountaineering, a field in which he had considerable expertise. In 1950 he led the first expedition to climb Tirich Mir, a 25,000-foot peak in the Hindu Kush in Pakistan.