
Ben: I can show you things. Things I know you want to see very badly. Let me put it so you'll understand. Picture a box. You know something about boxes, don't you, John? What if I told you that somewhere on this island, there's a very large box... and whatever you imagined, whatever you wanted to be in it, when you opened that box, there it would be. What would you say about that, John?
The subject of the novel is about a murder committed by the novel's anonymous narrator and his accomplice. They murdered a wealthy recluse, à la Charles Dickens' Scrooge, for his locked cash box, rumored to have an inordinate amount of money it. But the accomplice betrays the anonymous narrator, who then embarks on a search for it. This search leads him to a surreal police station where a pair of policemen "do not confine their investigations or activities to this world or to any known planes or dimensions."
Eventually, one of the policemen leads our protagonist to the 'box' of The Third Policeman universe. Here's the description, after the protagonist and the policeman take an elevator down to it:
I saw a long passage lit fitfully at intervals by the crude home-made noise-machines, with more darkness to be seen than light. The walls of the passage seemed to be made of pig-iron in which were set rows of small doors which looked to me like ovens or furnace-doors or safe-deposits such as banks have.
I shall not recount the passages we walked or talk of the one with round doors like portholes or the other place where the Sergeant got a box of matches for himself by putting his hand somewhere into the wall. It is enough to say that we arrived, after walking at least a mile of plate, into a well-lit airy hall which was completely circular and filled with indescribable articles very like machinery but not quite as intricate as the more difficult machines.
"Is this eternity?" I asked, "Why do you call it that?"
"We call it that," the Sergeant explained, "because you do not grow old here. When you leave here you will be the same age as you were coming in and the same stature and latitude."
To my astonishment he went over to one of the bigger ovens, manipulated some knobs, pulled open the massive metal door and lifted out a brand-new bicycle.
"What else is there?"
"Anything."
"Anything I mention will be shown to me?"
"Of course. Did you ever hear tell of omnium?"
"Omnium? And what is omnium the right name for?"
"You are omnium and I am omnium and so is the mangle and so are my boots..."
The ease with which the Sergeant produced the bicycle...had set in motion in my head certain trains of thought. Sitting at home with my box of omnium I could do anything, see anything and know anything with no limit to my powers save that of my own imagination. Perhaps I could use it even to extend my imagination. I would improve the weather to a standard day of sunny peace with gentle rain at night washing the world to make it fresher and more enchanting to the eye. I would present every poor labourer in the world with a bicycle made of gold.
My nervousness had been largely reduced to absurdity and nothingness by what I had seen and I now found myself taking an interest in the commercial possibilities of eternity. I ordered a bottle of whiskey, precious stones, some bananas, a fountain-pen and writing materials, and finally a serge suit of blue with silk linings.
"I am going to take these things with me," I announced.
"In that case you will need a big strong bag," the Sergeant said.
We smoked in silence and went on through the dim passage till we reached the [elevator] again. I was very tired with my bag of gold and clothes and whiskey and made for the [elevator] to stand on it and put the bag down at last. When nearly on the threshold I was arrested in my step by a call from the Sergeant...
"Don't go in there!"
"Why?"
"The bag, man. The simple thing is that you cannot enter the [elevator] unless you weigh the same weight as you weighed when you weighed into it. If you do, it will extirpate you unconditionally and kill the life out of you."
I understood little except that my plans were vanquished and my visit to eternity unavailing and calamitous.
Generally speaking, this all sounds very familiar to Ben's interrogation of Locke in "The Man from Tallahassee":
Ben: Are you afraid it'll go away, John? Is that why you want to destroy the submarine? Because you know if you ever leave this island you'll be back in the chair?
I'm sure we can all empathize with Locke's apparent dilemma. Who wouldn't want to keep the use of their legs and remain independently ambulatory? Locke doesn't seem to be able to contribute to humanity at large, at least not in the same way as, say, a spinal surgeon like Jack; so maybe Locke's desire to retain his mobility would not be considered selfish.
But what about Jack? Clearly he can do much good in the world with his singular skill as a surgeon, and in the Season 3 finale he was even able to respond to human suffering while in the throes of his own 'dark night of the soul' as he seemed to resign himself to suicide. Would it be selfish of Jack to remain on the island, assuming he also knew the island's secret powers that were vouchsafed to Locke by Ben? If one has the power to save lives, is one morally obligated to use that power? Is refraining from using one's life-saving power equivalent to outright killing someone? Would Jack be considered to be the most dispicable of human beings by choosing to stay on the island? Ben, always the devil's advocate, presents a starkly convincing picture for Jack in the Season 3 finale:

Ben: Let me ask you something, Jack. Why do you wanna leave the Island? What is it that you so desperately want to get back to? You have no-one. Your father's dead, your wife left you, moved on with another man. Can you just not wait to get back to the hospital? Get back to fixing things?
What would we do? Would we leave our comfortable lives, our lattes, our HDTV and spend our lives on the island, knowing that the unbelievable and unsurpassable things we could conjure up on the island must forever stay there?
Lost
