Because Bertrand Russell, A.J. Ayer and many others believed that no empirical statement (or “material-thing statement”) could ever be certain, one target for Russell himself were uses of the word ‘certain’ in ordinary language. As Norman Malcolm put it (or as he puts the “philosopher’s position”):
Malcolm, Norman. (1952/1964) 'Moore and ordinary language'
Moore, G.E. 'Certainty'