 What struggle does the door in the library represent?
When nobody aboard ship remembers where it's going, how can they tell when it has arrived? But ever since the engines somewhere in the rear of the world had changed their tone, from the steady whining Rikud had heard all twenty-five years of his life, to the sullen roar that came to his ears now, the feelings had grown. Instead, there was more of the blackness, and one very bright star set apart by itself in the middle of the viewport. Rikud knew that something which he could not explain had entered the elder man's head for a moment, but it had departed almost before Chuls knew of its existence. Once, here in this same ray room, he had had the impulse to hurl himself head-first against the wall, just to see what would happen. But something soft had cushioned the impact—something which had come into being just for the moment and then abruptly passed into non-being again, something which was as impalpable as air. There had been a revolt—again a term without any real meaning, a term that could have no reality outside of the reading machine—and the elders were overthrown. The people had decided that they did not know where they were going, or why, and that it was unfair that the elders alone had this authority. They were born and they lived and they died as the elders directed, like little cogs in a great machine. Much of this Rikud could not understand, but he knew enough to realize that the reading machine had sided with the people against the elders, and it said the people had won. "Yes?" Almost no one read any more, and the library was heavy with the smell of dust. You can find several of them under 'astronomy,' with a cross-reference under 'stars.' Something loomed there in the port, so huge that it spread out over almost the entire surface. A section of it had expanded outward and assumed the rectangular shape of the viewport, and its size as well. Instead an ominous silence, broken at regular intervals by a sharp booming. Rikud turned again to the port, where he could see nothing but an obscuring cloud of white vapor, murky, swirling, more confusing than ever. Now if the world had walked from—somewhere, through the vast star-speckled darkness and to the great garden outside, this also was purposeful. But if everyone lived as if the world still stood in blackness, how could they find the nature of that purpose?
 The struggle for Rikud and all the others to conceptualize what they don't know or haven't seen before.
 The struggle between man and machine, and the power machine now has over them.  The struggle for Rikud and all the others to conceptualize what they don't know or haven't seen before.  Rikud's fear of what's behind it.  The struggle between authority and the people it runs.