Celestia fretted around the tea set, making sure everything was in place. 

“Good morning, Princess! How are you?” she badly hid a startle, and turned to face the pony who she still thought of as her faithful student.
“Ah! Hello, Twilight. Please, please, sit”, she gestured to to the low table and the tea she'd brewed herself. Twilight looked at, her puzzled, before taking place in front of her.

“So, is there a particular reason you went to so much trouble to arrange this meeting, Princess? I'm sure you're quite busy with the fallout of Tirek's attacks, and I actually had to postpone the delivery of some furniture. Not that I mind, of course!”
Celestia grimaced.

“It's...a final lesson I still have to impart. I know you're not...my student anymore. But this lesson has become long overdue, and my” she sighed “dear sister has pointed out to me how remiss I have been in withholding this particular knowledge from you.”
The purple pony in front of her sipped from her cup. Celestia could tell she was trying not to show how eager she was to hear whatever she had to say.

Feeling like the world's biggest, clumsiest foal, she steeled herself for a moment and then started speaking again. 
“Four hundred years ago, Equestria was a police state.” “But Princess, that's!” “I know it seems impossible; you've read history books about that era, you've done reports on books written during that time. But I assure you, it's true.”

“What you're not taking into account is the pony race's propensity for forgetfulness, and how much time can distort any perspective. If you look at the statistics from that time, at the particular wording freedom of speech and privacy laws had in that era, you'll understand.”
Twilight was looking at her with her mouth open, her cup of tea forgotten in her hooves.

“But Princess, surely you weren't the one who-” the elder Princess cut her off: “I was the one who had let things degrade to such a state. I was the one who took away civil liberty after civil liberty, thinking I was better than ordinary ponies, that I could make their decisions for them, that their security mattered more than their freedom.”
“Eventually, there was a revolution, which ended with me killing its leader in front of everypony. Things...gradually became better after that. I became better.”

The purple pony princess in front of her now looked completely lost.
“W-why are you telling me all this?”
Celestia smiled ruefully. “It's not to make you lose faith in me, for sure. I know you've already noticed I can make mistakes just as well as anypony else-I shouldn't have ordered you to hide away while you had the power of all the other princesses, for example.”

Twilight blushed.
“No, it's for a much more fundamental reason. Twilight; you, me, Luna, Discord and others are all immortals. And one thing that happens to immortals, whether they want it or not, is change.”
“Fifty years from now, you may find yourself holding positions you would have never considered today. A hundred years from now, you may have very little in common with who you are today. And two hundred years from...you may be someone else entirely.”

“The lesson I want you to get from all this, is to always be mindful of who you may be turning into,  and of whether your past self would approve of your actions. Otherwise...you may wind up hating who you've become. Or who you were.”
A couple of awkward seconds of silence passed, while both princesses thought things over.
“I-I hope your opinion of me isn't too damaged, I just...” pleaded Celestia. “Don't worry, I'm still your friend.” barked Twilight “I just...I just have to think things over.”
“Ok.” 
“Actually, I have one last question: the leader of the...revolution; what were his last words?”
Celestia gulped. She'd hoped she wouldn't have to answer that, and she hadn't volunteered them because...well, Honesty had always been her sister's domain anyway. 
“I surrender.”