1. G. Kessler, Frege, Mill and the foundations of arithmetic, Journal of Philosophy 77 (1980), 65–79.
2. E.H. Reck, Frege on numbers: beyond the Platonist picture, Harvard Review of Philosophy 13 (3) (2005), 25–40.
3. A. Kenny, Frege (Penguin, Harmondsworth, 1995), 161.
4. S. Körner, The Philosophy of Mathematics: An Introduction (Harper & Row, New York, 1962).
5. M. Tiles, Mathematics and the Image of Reason (Routledge, London, 1991).