In logic, a three-valued logic (also trivalent, ternary, trinary logic, or trilean,[citation needed] sometimes abbreviated 3VL) is any of several many-valued logic systems in which there are three truth values indicating true, false and some indeter
Categorical Logic and Type Theory Bart JACOBS Research Fellow of the Royal Netherlands Academy of Arts and Sciences STUDIES IN LOGIC AND THE FOUNDATIONS OF MATHEMATICS VOLUME 141