Full Idea
It would be a vicious circle to define material implication as meaning that if one proposition is true, then another is true, for 'if' and 'then' already involve implication.
Gist of Idea
It would be circular to use 'if' and 'then' to define material implication
Source
Bertrand Russell (The Principles of Mathematics [1903], §037)
Book Reference
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.33
A Reaction
Hence the preference for defining it by the truth table, or as 'not-p or q'.