The pure calculus of entailment
Alan Ross Anderson, Nuel Belnap · Journal of Symbolic Logic · 1962
The “implicational paradoxes” are treated by most contemporary logicians somewhat as follows: “The two-valued prepositional calculus sanctions as valid many of the obvious and satisfactory inferences which we recognize intuitively as valid, such as (A→.B→C)→.A→B→.A→C,2 and A→B→.B→C→.A→C; it consequently suggests itself as a candidate for a formal analysis of implication.