"Contrariwise," continued Tweedledee, 
"If it was so, it might be  
and if it were so, it would be  
but as it isn't, it ain't. 
That's logic."


Tweedledum and Tweedledee
      Agreed to have a battle
For Tweedledum said Tweedledee
      Had spoiled his nice new rattle.
Just then flew down a monstrous crow,
      As black as a tar-barrel
Which frightened both the heroes so,
      They quite forgot their quarrel.

Tweedledee and Tweedledum are fictional characters in Lewis Carroll's Through the Looking-Glass, and What Alice Found There and in a nursery rhyme by an anonymous author. Their names may have originally come from "one of the most celebrated and most frequently quoted  epigrams", written by poet John Byro