“The Dark Arts," said Snape, "are many, varied, ever-changing, and eternal. Fighting them is like fighting a many-headed monster, which, each time a neck is severed, sprouts a head even fiercer and cleverer than before. You are fighting that which is unfixed, mutating, indestructible[...]"Your defenses," said Snape, a little louder, "must therefore be as flexible and inventive as the arts you seek to undo."