 Before his departure, Peter recalls a line from a film. Why does it come to mind for him?
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. 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 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. "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. 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. Peter sank back in his chair, the full shock of it striking him for the first time. " Peter took it in fifteen seconds, running, and stumbled to a halt in front of the door marked "Radiation." She had set her door mechanism to "Etaoin Shrdlu," principally because he hated double-talk. He mouthed the syllables, had to repeat them because he put an accent in the wrong place, and squeezed through the door as soon as it opened far enough to admit him. "What makes, Peter my love?" It was a street scene in the Science City of Manhattan, flooded by the warm spring sunshine.
 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.