Carl Gustav Hempel (2001). “The Philosophy of Carl G. Hempel: Studies in Science, Explanation, and Rationality”, p.24, Oxford University Press on Demand
George Bernard Shaw (2015). “George Bernard Shaw: Collected Articles, Lectures, Essays and Letters: Thoughts and Studies from the Renowned Dramaturge and Author of Mrs. Warren’s Profession, Pygmalion, Arms and The Man, Saint Joan, Caesar and Cleopatra, Androcles And The Lion”, p.305, e-artnow