1984, George Orwell

An oblong slip of newspaper had appeared between O’Brien’s fingers. For perhaps five seconds it was within the angle of Winston’s vision. It was a photograph, and there was no question of its identity. It was the photograph. It was another copy of the photograph of Jones, Aaronson, and Rutherford at the party function in New York, which he had chanced upon eleven years ago and promptly destroyed. For only an instant it was before his eyes, then it was out of sight again. But he had seen it, unquestionably he had seen it! He made a desperate, agonizing effort to wrench the top half of his body free. It was impossible to move so much as a centimetre in any direction. For the moment he had even forgotten the dial. All he wanted was to hold the photograph in his fingers again, or at least to see it.

“It exists!” he cried.

“No,” said O’Brien.

He stepped across the room.

There was a memory hole in the opposite wall. O’Brien lifted the grating. Unseen, the frail slip of paper was whirling away on the current of warm air; it was vanishing in a flash of flame. O’Brien turned away from the wall.

“Ashes,” he said. “Not even identifiable ashes. Dust. It does not exist. It never existed.”

“But it did exist! It does exist! It exists in memory. I remember it. You remember it.”

“I do not remember it,” said O’Brien.

Winston’s heart sank. That was doublethink. He had a feeling of deadly helplessness. If he could have been certain that O’Brien was lying, it would not have seemed to matter. But it was perfectly possible that O’Brien had really forgotten the photograph. And if so, then already he would have forgotten his denial of remembering it, and forgotten the act of forgetting. How could one be sure that it was simple trickery? Perhaps that lunatic dislocation in the mind could really happen: that was the thought that defeated him.

O’Brien was looking down at him speculatively. More than ever he had the air of a teacher taking pains with a wayward but promising child.

“There is a Party slogan dealing with the control of the past.” he said. “Repeat it, if you please.”

“Who controls the past controls the future; who controls the present controls the past.” repeated Winston obediently.

---source : 1984, George Orwell.

This passage may be a long one hard to understand.

A question for this passage is like this.

Q. What is the main idea of the passage?

Answer : controlling understanding of past events gives people power.

Do you happen to know the meaning of "controlling understanding of past events gives people power."?