  I am grateful to Fabrizio Cariani, Kosta Došen, Solomon Feferman, Mario Gómez-Torrente, Graham Priest, Greg Restall, Gila Sher, and an anonymous reviewer for comments that helped improve this entry. Parts of the entry are derived from chapters 1, 2, 3, and 6 of my dissertation, “What Does It Mean to Say that Logic is Formal?” (University of Pittsburgh, 2000).