Clifford var en kraftig motstander av obskurantistiske tankeretninger. Et av hans mest kjente sitater er <ref> 1877: Mind-stuffTribal selfElements of DynamicsSeeing and thinkingCommon sense of the exact sciencesLectures and essays by the late William Kingdon CliffordEthics of belief and other essaysConditions of mental development, and other essaysSelected worksMathematical PapersMathematical fragments, being facsimiles of his unfinished papers relating to the theory of graphsWilliam CliffordWilliam Kingdon Clifford (1845–1879) and his wife Lucy (1846–1929)Such Silver Currents – The Story of William and Lucy Clifford, 1845–1929.'' Cambridge, The Lutterworth Press, 2002. ISBN 0-7188-3017-2.