"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."
"I know what you're thinking about," said Tweedledum: "but it isn't so, nohow."
"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."
Lewis Carroll, "Through the looking-glass" (Chapter IV).