 Before his departure, Peter recalls a line from a film. Why does it come to mind for him?
It was I who had changed: something inside me was dead, like the machinery, and like Peter. It was a kind of indefinable emptiness. I do not think it was what Peter called an emotion; and yet it had nothing to do with logic, either. It was just an emptiness—a void that could not be filled by eating or drinking. I did not even wish that Peter were not dead, for reason had told me that he had to die. But the void was still there, unexplainable and impossible to ignore. His dream was perfect; the Citadel was complete, every minutest detail provided for—on paper. Then there would remain only the task of blasting the great, shining ship out into the carefully-calculated orbit that would be its home. He could see, too, the level on level of laboratories and storerooms that filled its interlocking segments; the meteor deflectors, the air renewal system, the mighty engines at the stern—all the children of his brain. The ship would be filled with the sounds of busy men and women, wresting secrets from the reluctant ether. But the other part was going calmly, lucidly on, quite without his volition. He stared at them dully, and then he reached over to the newsbox on his desk, and switched it on. The order was made public early this morning by R. Stein, Secretary of the Council, who said in part: "The reason for this ... order must be apparent to all civilized peoples. There can be few within reach of printed reports or my words who have not seen the Invaders, or whose friends have not seen them. "The peoples of the world, then, know what they are, and know that we face the most momentous struggle in our history. "Since the Invaders first appeared in Wood River, Oregon, 24 hours ago, they have not once acknowledged our attempts to communicate, or in any way taken notice of our existence as reasoning beings. They have treated us precisely as we, in less enlightened days, might have treated a newly-discovered race of lower animals. But in instance after instance, they have done as they would with us. They have examined us, dissected us, driven us mad, killed us with no discernable provocation; and this is more intolerable than any normal invasion. "I have no fear that the people of Earth will fail to meet this challenge, for there is no alternative. Not only our individual lives are threatened, but our existence as a race.
 He recognizes that he will be a changed, mutated man when he returns. He literally will come back "not as a boy."
 He recognizes that he will be a changed, mutated man when he returns. He literally will come back "not as a boy."  He's trying to convince himself that he and humanity will be able to come back, with the emphasis on "We'll come back."  The situation is grave. Like men who go off to war, the journey will change them. He won't be coming home as the same "boy."  He's not sure he'll be coming back, and the song is bittersweet for him.