 In 1962 he became Roy Roberts Distinguished Professor of Philosophy at the University of Kansas, and in 1966 Professor of Philosophy at Northwestern University, where he was later named John Evans Professor of Moral and Intellectual Philosophy and where he taught until his retirement in 1976.At the World Congress of Philosophy in Vienna in 1968, he chaired the meeting that established the International Society for Metaphysics.After retirement he taught as a visiting Professor at Carleton College, Marquette, Villanova (as Distinguished Professor of Christian Philosophy) and Emory Universities and was an honorary research fellow at the Centre for Philosophy and History of Science at Boston University.