1. Bell, J. L. (1994): Fregean Extensions of First-order Logic, Mathematical Logic Quarterly 40: 27–30.
2. Bell, J. L. and Machover, M. (1977): A Course in Mathematical Logic, North-Holland, Amsterdam.
3. Carnap, R. (1937): The Logical Syntax of Language, transl. by A. Smeaton, Harcourt, Brace, New York.
4. Carnap, R. (1957): Introduction to Symbolic Logic and its Applications, transl. by W. H. Meyer and J. Wilkinson, Dover, New York.
5. DeVidi, D. and Solomon, G. (1995): Tolerance and Metalanguages in Carnap's Logical Syntax of Language, Synthese 103: 123–139.