 In 1971, he published a third edition of his Locke biography and the book "Knowing and the Function of Reason", which includes a broad discussion of the laws of non-contradiction, excluded middle and identity, of the use of language in speech and thought, and of substance and causality.After retiring in 1969, he taught for a semester at Carlton College in Minnesota before returning to Wales, where he.helped write articles for the 1974 edition of the "Encyclopædia Britannica".