In his talk this morning, Grisha Mints referred to a paper by Paul Cohen. He didn’t have the reference handy, so I tracked it down:
Paul J. Cohen, Skolem and pessimism about proof in mathematics. Phil. Trans. R. Soc. A (2005) 363, 2407–2418.
The entire issue, on meeting on “The nature of mathmatical proof” organized by Alan Bundy in 2004, is of interest:
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences (Volume 363, Number 1835 / October 15, 2005)