"Princess," the letter finished. "You'd better come quickly. She hasn't slept in three days."

Celestia put down the scroll, told the guards to cancel the remainder of the day court, and teleported herself to Ponyville.

"Oh thank Celestia... err... you," Spike stammered.

"How bad is it, Spike?"

"See for yourself," he said as he pointed to the top of the rolling library ladder. Atop it perched a frazzled Twilight Sparkle, who levitated half a dozen books around herself and was plucking more from the shelves as the ladder slid around the massive room.

"Twilight?" Celestia ventured.

The librarian looked up, eyes darting around frantically, before looking down and seeing Celestia. "Oh, Princess!" she said, teleporting herself to the ground instantly, leaving the previously floating books to clatter to the ground behind her.

"What's wrong, Twilight?"

"It was a... a dream. Or it seemed like a dream, but maybe it wasn't. It seemed real, and it was silly, but then it wasn't silly because it was real!"

"Calm down, and tell me what happened."

Taking a deep breath, Twilight explained. "I remember waking in a windowless room. There was a bed, and food, and water. A comfortable prison. Then a voice sounded from above, but I couldn't see the source. It explained this was a test."

"A test?" Celestia said. Her former student didn't have a good track record when dealing with surprise tests.

"Yes, a test. For our whole race. It said I'd been chosen to represent everypony, and another being of some kind—it wouldn't tell me what—was representing theirs. All we had to do was talk to each other. But that's all we could do. Talk. There was some sort of magic or machine that carried my voice to it and its to me. But nothing else."

"So what did you talk about?"

"That's just it, we couldn't. At least not at first. I tried every language I knew, but it didn't know any of them. And I had no idea what it was saying. So I started with first principles: tapped out basic concepts like zero and one, yes and no, and over the weeks and months, we started to communicate."

"That seems like a very difficult thing to do," Celestia said, starting to see how this dream could really stress a mind like Twilight's.

Twilight let loose a nervous chuckle. "Actually," she said. "I've thought a lot on that sort of thing, and it's not as hard as you think. It just takes time. Months I think in this case. Whatever it was, it was smart too, and we were really starting to talk by the end of it. We hadn't heard the other voice in a long time, but then it spoke up. It told us we were to prepare for the final test."

"And?"

"An identical, bilaterally symmetrical panel appeared in the wall of each cell. And I do mean symmetrical. No screws or other sign of chirality anywhere. We looked! Each panel had two buttons on it, also identical. One on the left, and one on the right. The voice told us all we had to do to pass was to press the same button. But if we pressed opposite buttons, both our worlds would be destroyed."

"So did you say left or right?"

"That's just it. I couldn't say left or right!"

"Dreams can be strange like that."

"No! You can't say left or right because they don't mean anything!"

"Sure they do," Spike interrupted. "Right claw," he pointed. "Left claw."

"But how do you know which is which? How can you say definitively what is left and what is right, without pointing or touching or otherwise showing it?"

"Umm..." Spike hesitated.

"You can't!" Twilight yelled, wings fluffing in frustration. "You can't say it! No pony can. It's never been said! Ever! I checked!"

"But surely there's a definition somewhere?" Celestia said.

"Nope!" Twilight said, becoming more manic. "Up is easy. 'Away from gravity.' Forward? 'The direction of travel.' Down and back? Just opposites. But left or right? Made up! Arbitrary! Impossible to explain!"

"That can't—"

"No, seriously! At the end of time, when everything that can happen has, the only thing in the universe left unsaid will be the difference between left and right!"

"So what did you do?"

"I asked for a recess. It said I could return home for 67 hours of research."

"Spike," Celestia said, becoming nervous. "How long has she been awake?"

"About 66 hours."