6E Zermelo. Investigations in the Foundations of Set Theory [M]. Ⅰ. Translated by S Bauer-Mengelberg, in J van Heijenoot, (ed.). From Frege to Godel, Harnard University Press,1967. 200.
7S H aaek. Philosophy of Logics[M]. London:Cambridge University Press,1978. 138-139.