A professor of philosophy at the University of California, San Diego (1963–73), and Washington University, St Louis, Missouri (1973–86), Richard Popkin was best known for his work on scepticism, notably for his classic study The History of Scepticism from Erasmus to Descartes (1960). This revolutionised the received picture of both the history of philosophy and the history of science, by demonstrating the influence, in the century before Descartes, of ancient Greek sceptical arguments, deriving ultimately from Pyrrho, about the impossibility of knowing God and the world. Instead of treating the history of science and philosophy as a series of breakthroughs by canonical figures, Popkin viewed the thought of the past from within its own framework.

He expanded his thesis in later editions of the book (most recently in 2003) and in The High Road to Pyrrhonism (1989), which took the story through to David Hume. His interest in the contribution of non-philosophical strands (especially religion) to the history of philosophy led to pioneering studies of the interaction of Jewish and Christian philosophy and theology, and of topics such as kabbalism and millenarianism. He opened up new perspectives, both on littleknown figures such as the French millenarian Isaac la Peyrère and the English Bible scholar Joseph Mede, and on major figures, especially Spinoza and Newton. He played a major role in generating interest in Newton’s legacy of non-scientific manuscripts.

Popkin also achieved fame with The Second Oswald (1966), in which he disputed the findings of the Warren Commission that a lone assassin killed President Kennedy. He foresaw the rise of religious fundamentalism in the United States and the Middle East, contributing an analysis of its American dimension in Messianic Revolution (1998, co-authored with David Katz). He also wrote for a general philosophical readership, with such books as Philosophy Made Simple (1969, co-authored with Avrum Stroll).

Richard Henry Popkin, philosopher, born Manhattan, New York, 27 Dec 1923; died Pacific Palisades, California, 14 April 2005, aged 81.