"You have visions, in your sleep, do you not? On occasion at least."
Celestia did not answer immediately. And not just because of the physical discomfort the harness she was hanging from gave her. "I do," she finally admitted, and by the tone in which she did she seemed to be expecting some kind of long and likely painful lecture or discussion about the matter.
It did not come, though the look in Twilight's eyes made it very clear that it would, eventually. Instead, the younger alicorn asked, "How do you tell one apart from a dream?"
That caught Celestia by surprise. Enough so that she spoke with no more discomfort, but genuine and almost motherly concern, as she asked, "Why?"
"I saw something, yesterday night," said Twilight. "It's part of the reason I called you here, this discussion. A lone unicorn in a house, somewhere in the woods, not like any place I've seen in Equestria. There was something wrong with her, I could feel it, but I couldn't place what it was. And she felt... distant." Twilight looked straight at Celestia. "Not physically distant. It's not something I've ever experienced before, but she felt distant on a temporal level. But dreams can be weird like that, I can't tell if I'm putting too much weight on something that shouldn't have it."
Celestia swallowed. "Did you ask Luna about the matter? You should-"
"The morning after," Twilight replied. "It was the first thing I did, and the second was write down the most detailed description of what I'd seen I could muster, so it wouldn't be lost as I forgot. By the time I was done, she'd answered." She began to pace around Celestia's suspended body. "She said she hadn't been patrolling anywhere close to my dreams, that night, and there was no checking anymore. So I came to you." She stopped behind Celestia, where she could not be seen.
Celestia forced herself to slow her breathing. "I would like to look over your notes later. I could find something more useful in them, perhaps some signs you haven't noticed." She swallowed again. "You can develop a feeling for what is and isn't a vision, but it takes time and experience. However, if this was as far in the future as you felt it was, there may simply be nothing to do. Nothing but wait, at least."
"I see," Twilight said, her tone going cold. "Well, then. Shall we begin?"