A certain speech by the protagonist in Planescape: Torment. Details below, but spoilered just in case.
The game's central question, asked by a number of people, is "what can change the nature of a man?" The main character, an immortal, asks his lost mortality (which is now sentient and is in no hurry to merge with him) this, to which it replies that the question is pointless. He insists, and gets the answer "Then this is my answer and you are the proof: NOTHING can change the nature of a man." Then comes the counter.
"If there is anything I have learned in my travels across the Planes, it is that many things may change the nature of a man. Whether regret, or love, or revenge or fear - whatever you believe can change the nature of a man, can. I?ve seen belief move cities, make men stave off death, and turn an evil hag's heart half-circle. This entire Fortress has been constructed from belief. Belief damned a woman, whose heart clung to the hope that another loved her when he did not. Once, it made a man seek immortality and achieve it. And it has made a posturing spirit think it is something more than a part of me."
A couple references to the game itself are lost on anyone who hasn't played it, I know. Still, of all the excellent lines in the game, that speech stuck with me.
"If there is anything I have learned in my travels across the Planes, it is that many things may change the nature of a man. Whether regret, or love, or revenge or fear - whatever you believe can change the nature of a man, can. I?ve seen belief move cities, make men stave off death, and turn an evil hag's heart half-circle. This entire Fortress has been constructed from belief. Belief damned a woman, whose heart clung to the hope that another loved her when he did not. Once, it made a man seek immortality and achieve it. And it has made a posturing spirit think it is something more than a part of me."
A couple references to the game itself are lost on anyone who hasn't played it, I know. Still, of all the excellent lines in the game, that speech stuck with me.