Harry grew frustrated at this. "I only did that in order to find and destroy the-" Harry's mouth shut quickly and against his will. His mind was racing, looking for something, anything else to ponder about. He fought the urge and thought about what he was trying to not think about. I cannot say "Horcruxes'? Harry mused, realizing that the Unspeakables were essentially attempting to restrict access to such knowledge. It's like a "Notice-me-not" charm, but about magic, about thinking about it... Harry didn't like the implications of that.