Rule of Material Implication/Definition

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The Rule of Material Implication can be used as a definition for the logical operator that is implication:

$p \implies q := \neg p \lor q$

Historical Note

This definition appears in the introductory material of 1910: Alfred North Whitehead and Bertrand Russell: Principia Mathematica: Volume $\text { 1 }$, and some other authors follow their lead.