On the independence of Robinson's set of axioms for propositional calculus

Abstract

We give a normal five-valued truth-table proving independence of one of the axioms in Robinson's set of axioms for propositional calculus from 1968, answering a question raised in his article, where he uses a non-normal truth-table. We also give a normal four-valued truth-table proving independence of one of the other axioms, where he uses a normal five-valued truth-table.