From 1936, he was a lecturer in philosophy at Bangor, becoming professor in 1947. In 1955, he was appointed Professor of the History and Philosophy of Religion at the University of London, retiring in 1977. He published in English and Welsh. (He was a regular attender at the National Eisteddfod of Wales and was a member of the Gorsedd of the Bards under the bardic name of Hywel Athronydd.) He had three main philosophical interests, political philosophy, ethics and the philosophy of religion. His interest in comparative religion led to his becoming founding editor of the journal Religious Studies, holding the post from 1964 to 1979. He edited the Muirhead Library of Philosophy from 1947 to 1978. He also served as chairman of the council of the Royal Institute of Philosophy from 1965 to 1968.
He delivered many of the most prestigious named lectures, among them the Gifford Lectures in Edinburgh (1966-68), the Wilde Lectures at Oxford (1960-63), the Hobhouse in London and the Owen Evans at Aberystwyth (1964-65). He lectured and was visting professor at numerous universities in the United States, Canada, India and Japan including Toronto, Boston, Philadelphia, Havard and Madras.He was awarded an honorary D.D. by St. Andrews University (1964) and a D.Litt. by Emory University in the U.S.A. (1978). He married Megan Jones 17 August 1943 but she died in 1962. He married for the second time Megan Pritchard 17 July 1965.