 Over the years, many readers and a few wise and conscientious referees have offered corrections and improvements to this entry. I am still grateful to Edward Horton (for pointing out that replacing ex falso by the LEM in the axioms for \(\mathbf{IPC}\) does not yield all of \(\mathbf{CPC},\) and for providing the correct substitutions) and to all the other readers who have corrected errors in earlier editions. I thank Mark van Atten, Robert Thomas, Victor Pambuccian, Michael Beeson, Mariusz Stopa and Antonino Drago for bringing new and old work to my attention since the last revision. Questions from students are always appreciated; this time, Miles Shi’s question led to an improvement in Section 5. As always, I thank Ed Zalta for his patience and attention to detail, and for the very existence of this comprehensive open-access encyclopedia.