Albus Dumbledore: [after Harry confusedly finishes viewing a memory from Dumbledore's pensieve] Confused? I'd be surprised if you weren't.
Harry Potter: Well, I don't understand. What happened?
Albus Dumbledore: This is perhaps the most important memory I've collected. It is also a lie. This memory has been tampered with by the same person whose memory it is, our old friend, Professor Slughorn.
Harry Potter: But why would he tamper with his own memory?
Albus Dumbledore: I suspect he's ashamed of it.
Harry Potter: Why?
Albus Dumbledore: Why, indeed? I asked you to get to know Professor Slughorn, and you have done so. Now I want you to persuade him to divulge his true memory any way you can.
Harry Potter: I don't know him that well, sir.
Albus Dumbledore: This memory is everything. Without it, we are blind. Without it, we leave the fate of our world to chance. You have no choice. You must not fail.