Willard Van Orman Quine, Dagfinn Føllesdal, Douglas B. Quine (2008). “Confessions of a Confirmed Extensionalist: And Other Essays”, p.289, Harvard University Press
Willard Van Orman Quine, Dagfinn Føllesdal, Douglas B. Quine (2008). “Confessions of a Confirmed Extensionalist: And Other Essays”, p.341, Harvard University Press