A reduction in number of the postulates for C. I. Lewis’ system of strict implication
J. C. C. McKinsey · Bulletin of the American Mathematical Society · 1934
He allows the following operations : Substitution, (a) Either of two equivalent expressions may be substituted for the other, (b) Any proposition, or any expression which has meaning in terms of the undefined ideas, may be substituted for p, or g, or r, etc., in any assumption or established theorem. Adjunction. If p has been asserted and q has been asserted, then pq may be asserted. Inference. If p has been asserted and p<q has been asserted, then q may be asserted.