What is David’s curious understanding with Rosalind?

David and Rosalind agreed that they would never discuss their dreams with other people.

She shook her head, and told me that there was no such place — not now. But, perhaps, she suggested, I could somehow be dreaming about times long ago. Dreams were funny things, and there was no accounting for them; so it might be that what I was seeing was a bit of the world as it had been once upon a time — the wonderful world that the Old People had lived in; as it had been before God sent Tribulation.

But after that she went on to warn me very seriously not to mention it to anyone else; other people, as far as she knew, did not have such pictures in their heads, either sleeping or waking, so it would be unwise to mention them.