1. What Numbers Could not Be
2. Boolos George [1984]: ‘To be is to be the value of a variable (or to be some values of some variables)’, in Richard Jeffrey, ed., Logic, Logic and Logic, pp. 54–72. Harvard University Press.
3. Dedekind Richard [1932]: Gesammelte Mathematische Werke, Vol. III. R. Fricke, E. Noether and Ø. Ore, eds. Brunswick: Vieweg.
4. Frege Gottlob [1984]: ‘On Mr. H. Schubert's numbers’, in Brian McGuinness, ed., Collected Papers on Mathematics, Logic and Philosophy, pp. 249–272. New York: Basil Blackwell.