[It's almost reminiscent of when Edward deduced Batman's secret identity---it's a riddle he literally can't share. A riddle that everyone knows the answer to is worthless, after all. And yet... even without Edward's compulsion, even if he could share the answer to this riddle, he simply doesn't want to.]

I'm a man of my word. [A brief pause, before he continues:] Look. You've shown me first-hand what it feels like to hear voices... thoughts that aren't even your own. Maybe I don't truly know what it's like to endure what you had to endure. I don't---[Choose your words carefully, Edward. Make an effort. He's faced torture, hasn't he? Not to the same degree as Schuldig, but...] Well. All right. Perhaps I still don't fully understand you.

...but I know what it's like to feel alone. I know what it's like to hate the world. And... to hate yourself, too.