John Corcoran and Matt Lavine, "Discovering string theory". Bulletin of Symbolic Logic. 19 (2013) 253–4.
Alonzo Church, Introduction to Mathematical Logic, Princeton UP, Princeton, 1956
John Corcoran, William Frank and Michael Maloney, "String theory", Journal of Symbolic Logic, vol. 39 (1974) pp. 625– 637 /wiki/John_Corcoran_(logician)
Pages 173–4 of Alfred Tarski, The concept of truth in formalized languages, reprinted in Logic, Semantics, Metamathematics, Hackett, Indianapolis, 1983, pp. 152–278