Independence of two nice sets of axioms for the propositional calculus
T. Thacher Robinson · Journal of Symbolic Logic · 1968
Kanger [4] gives a set of twelve axioms for the classical prepositional calculus which, together with modus ponens and substitution, have the following nice properties: (0.1) Each axiom contains =⊃, and no axiom contains more than two different connectives. (0.2) Deletions of certain of the axioms yield the intuitionistic, minimal, and classical refutability1 subsystems of propositional calculus.