Hilary Putnam 
Philosophy of Logic (Routledge Revivals) [PDF ebook] 

First published in 1971, Professor Putnam»s essay concerns itself with the ontological problem in the philosophy of logic and mathematics – that is, the issue of whether the abstract entities spoken of in logic and mathematics really exist. He also deals with the question of whether or not reference to these abstract entities is really indispensible in logic and whether it is necessary in physical science in general.