p. 169

“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 and indestructible….

“Your defences,” said Snape, a little louder, “must therefore be as flexible and inventive as the Arts you seek to undo.”

p. 415

Voldemort boasted:

“…I have pushed the boundaries of magic further, perhaps, than they have ever been pushed–“

“Of some kinds of magic,” Dumbledore corrected him quietly. “Of some. Of others, you remain…forgive me…woefully ignorant.”…

…”The old argument,” he said softly. “But nothing I have seen in the world has supported your famous pronouncements that love is more powerful than my kind of magic, Dumbledore.”

“Perhaps, you have been looking in the wrong places,” suggested Dumbledore.