Frege 1884, §5: "Kant declares [statements such as 2 + 3 = 5] to be unprovable and synthetic, but hesitates to call them axioms because they are not general and because the number of them is infinite. Hankel justifiably calls this conception of infinitely numerous unprovable primitive truths incongruous and paradoxical." Frege, Gottlob (1884). Die Grundlagen der Arithmetik. Eine logisch-mathematische Untersuchung über den Begriff der Zahl. Breslau: Verlag von Wilhelm Koebner.
Frege 1884, §14: "The fact that [denying the parallel postulate] is possible shows that the axioms of geometry are independent of one another and of the primitive laws of logic, and consequently are synthetic. Can the same be said of the fundamental propositions of the science of number? Here, we have only to try denying any one of them, and complete confusion ensues." Frege, Gottlob (1884). Die Grundlagen der Arithmetik. Eine logisch-mathematische Untersuchung über den Begriff der Zahl. Breslau: Verlag von Wilhelm Koebner.
Frege 1960, p. 10: "If the definition of each individual number did really assert a special physical fact, then we should never be able to sufficiently admire, for his knowledge of nature, a man who calculates with nine-figure numbers." Frege, Gottlob (1960). The Foundations of Arithmetic: A Logico-Mathematical Enquiry Into the Concept of Number. Translated by Austin, J. L. (2nd ed.). Evanston, Illinois: Northwestern University Press. ISBN0810106051. OCLC650.
Frege 1960, p. 11: "[...] the number 0 would be a puzzle; for up to now no one, I take it, has ever seen or touched 0 pebbles." Frege, Gottlob (1960). The Foundations of Arithmetic: A Logico-Mathematical Enquiry Into the Concept of Number. Translated by Austin, J. L. (2nd ed.). Evanston, Illinois: Northwestern University Press. ISBN0810106051. OCLC650.
Boolos 1998, p. 154: "Frege defines 0 as the number of the concept: being non-self-identical. Since everything is self-identical, no object falls under this concept. Frege defines 1 as the number of the concept being identical with the number zero. 0 and 0 alone falls under this latter concept." Boolos, George (1998). "Chapter 9: Gottlob Frege and the Foundations of Arithmetic". Logic, logic, and logic. Edited by Richard C. Jeffrey, introduction by John P. Burgess. Cambridge, Mass: Harvard University Press. ISBN9780674537675. OCLC37509971.