McCulloch W. S. (1960) What is a number, that a man may know it, and a man, that he may know a number? General Semantics Bulletin 26/27: 7–18. Available at https://cepa.info/1821McCulloch W. S. (1960) What is a number, that a man may know it, and a man, that he may know a number? 
Not sure if the link to pdf above will render in WordPress so here it is again: http://www.vordenker.de/ggphilosophy/mcculloch_what-is-a-number.pdf
Lots of very nice personal reminiscences (including of Pitts) and some sharp summaries, a review of philosophy and relevant science to date.