Carl Gustav Hempel (2001). “The Philosophy of Carl G. Hempel: Studies in Science, Explanation, and Rationality”, p.13, Oxford University Press on Demand
Charles Sanders Peirce, Nathan Houser, Christian J.W. J. W. Kloesel (1992). “The Essential Peirce, Volume 1: Selected Philosophical Writings? (1867–1893)”, p.56, Indiana University Press