H. P. Grice on A. N. Whitehead's and B. A. W. Russell's Material Implication

Source : Principles : Chapter III. Implication and Formal Implication. –  Source : Principia, page 7 : "When it is necessary explicitly to discriminate "implication" [i.e. "if p, then q" ] from "formal implication," it is called "material implication." – Source : Principia, page 20 : "When an implication, say ϕx.⊃.ψx, is said to hold always, i.e. when (x):ϕx.⊃.ψx, we shall say that ϕx formally implies ψx"