The existence of mail does not imply the existence of a mailman. Mail can be explained in other, less supernatural, ways.
Nor does a letter protesting about the monster imply the existence of the monster. The protest (no matter how strongly worded) only has force if the monster exists. If there is no monster, the protest lapses, but if there is a monster, Elvis has covered that contingency.
(“Imply” in the sense of strict material implication, such that p → q is equivalent to not (p or q). Since GoComics does not seem to fully support classical notation, Cpq equivalent to NKpq in Polish notation.)
February 26, 2016