Full Idea
The paradoxes of material implication are P |- Q → P, and ¬P |- P → Q. That is, since Napoleon was French, then if the moon is blue then Napoleon was French; and since Napoleon was not Chinese, then if Napoleon was Chinese, the moon is blue.
Gist of Idea
The paradoxes of material implication are P |- Q → P, and ¬P |- P → Q
Source
E.J. Lemmon (Beginning Logic [1965], 2.2)
Book Reference
Lemmon,E.J.: 'Beginning Logic' [Nelson 1979], p.60
A Reaction
This is why the symbol → does not really mean the 'if...then' of ordinary English. Russell named it 'material implication' to show that it was a distinctively logical operator.