In the middle of a full moon night, Owlowiscious' insistent hooting called Princess Twilight Sparkle to her library.
She found her books on magic, illusions, and quantum theory, and her notes all scattered about the floor: including her notes from her human world visits (she was pretty sure she'd stored them under lock and key, with the key safely sealed in a pocket dimension only Twilight knew the harmonic frequency to), and her reminder to herself to explain to Pinkie Pie that random distractions (like juggling demonstrations) during a long philosophical friendship lecture would be, well, distracting.
Her first instinct was to reorganize her beloved books on the spot... but the now calm Owlowiscious pointed and she saw Trixie on the castle balcony, staring up at Luna's moon as if the rest of the world didn't exist.
Twilight thought, 'Okay, if Trixie sprouts fangs while looking at the moon, teleport to the lab and throw together some 'anti-werepony' formula.'
Trixie didn't seem to notice as Twilight carefully trotted close to her. Finally, Trixie spoke, "I see... I see at last... It is neither dream, nor lie, nor figment, but it is only shadows."
Concerned (and confirming there was no Alicorn Amulet around Trixie's neck), Princess Twilight Sparkle asked, "Trixie, are you okay?"
Trixie answered in a calm voice, still looking at the moon, "Twilight... I understand now... We are spirits, wandering a world of illusions. Only we are real. 'I think therefore I am', René Descartwheel was more right than he thought! It's all shadows Twilight! Illusions!" Trixie suddenly turned to face Twilight, her front hooves outstretched. "WE HAD IT ALL BACKWARDS! We thought the spiritual couldn't exist without the physical's belief in it! It's the spiritual that's real, it's all this that's a trick!"
Twilight tilted her head. "Uh, Trixie, are you talking about us being trapped in some giant illusion spell? Because I think Sunset Shimmer mentioned a movie like that in the human world-"
"No, Twilight! I'm not! Not at all! This isn't an artifactual construct in our minds being held in a REAL physical world, the physical universe ITSELF is the illusion! Our spirits are blinded by this!"
"Ohh... kay." Twilight inched herself between Trixie and the balcony. "Trixie, this isn’t like you... are you feeling hurt? Are you suffering from depression? Did you think nopony loves you? Do you think you have to escape from reality in order to-"
Trixie reared her head back a bit in confusion. "What are you talking about, Sparkle?"
Twilight said encouragingly, "Just that you shouldn't think you have to give up on the world."
Trixie noticed how Twilight had firmly positioned herself between Trixie and the nearest vertical drop. "I'm not suicidal Twilight! Stop hearing what you want to hear and LISTEN! I've learned more and more about illusion magic! I've dived into it, and I've realized, illusion magic and 'real' magic have no actual difference! It's just one illusion over another!"
Twilight's eyes grew large with sympathy. And her horn readied a powerful stun spell just in case. "Trixie... I think you've... gotten confused. I think you've learned so much about magic so fast, and I want you to know I'm very proud of you and of Starlight for being able to teach you, but you've gotten too engrossed, and gotten illusions and reality muddled. How about we go inside and have a nice cup of tea, I know some ponies who can-"
"And I'm not crazy either!" Trixie snapped, stomping a hoof. She took a deep breath, stepped back, and tried to speak more calmly. "Everything we ponies physically experience is just shadows. Electrical signals interpreted by the brain from organs detecting light, vibration, and so on. But none of that is 'real', because everypony's brain is slightly different. Who's to say my blue is the same as your blue?"
Twilight awkwardly tapped her front hooves together. "Uh, there have been discussions of that before, but that doesn't mean nothing is real."
Trixie summoned up a diagram that was... poorly drawn. "Twilight! I just said that WE are real! Our consciousness, our minds are real! It's all the rest that are illusions."
"Trixie, you're not making sense!" Twilight unconsciously assumed the 'Princess Luna explains things' pose. "If everything is an illusion, then how are you breathing illusionary air? How do you feel pain when you bump your knee? Because that's an illusion too? You're using circular logic!"
Trixie sighed and turned away. "You really don't get it. I... I don't blame you. After all, I'm the pony who spent her life deceiving others and deceiving myself right?"
Twilight deflated a little. "Trixie, I promise, this isn't about that. I think maybe you're overtired, and you're not thinking this through-"
"I have thought it through, Twilight, over and over! I'm not saying this is all a collective delusion, or that it's a manufactured experience." Trixie did a dramatic flourish, doing a three-sixty-spin, her mane and tail swirling behind her. "I'm saying that none of it is REAL!" Trixie went to her diagram again, showing little ponies in a big thought bubble the ponies INSIDE the bubble were 'dreaming'... or was it something else doing the dreaming? The picture Trixie pointed to could have also been an illustration of parasprites doing a rendition of the Dance of the Sugar Plum Breezie. "Things can be really hotter or colder right? I'm saying that our spiritual presence is 'more real' than this transient material universe!"
"Ugh, okay! Fine! So we're...characters in an enchanted comic then? We're figments of somepony's dream? A little filly playing pretend with her dolls?"
"UGH!" Trixie slapped her hoof into her own forehead. "I told you to listen to me! I... I... I'm sorry! I'm a great storyteller, but I just don't know THE WORDS to explain it all to you!"
Twilight slowly shook her head sadly. "Trixie, you're describing Plate Oats' cave. It's a thousand-year old analogy about how we agree on things via consensus about our senses. But that doesn't mean there's nothing there."
"Twilight... there isn't 'nothing'. I'm saying this world is the phantom. Our spirits, they're the only thing that's actually real. Just because something is not perceived, does not mean it does not exist. I think maybe it's because I approached everything from the other way around. Instead of learning magic and then illusions, I learned illusions and then magic. And it hit me, the illusions I place on ponies' minds, is no different from the 'illusions' regular magic places on reality."
Twilight shook her head, and stepped closer to Trixie. "Trixie, that's just the difference between 'permanent' magic or 'spontaneous' magic. The difference between magic that can be dispelled, and magic that alters things' natural state. Like casting a dispel magic won't turn a pile of ashes back into a tree after it was hit by a magic fireball. But it would undo a breezie transformation spell."
"Twilight, I'm a pro at illusion magic, you agree, yes? Yes. Then listen! With enough skill, power, and guile, you can place one illusion on top of another, AND you can sometimes alter SOMEONE ELSE'S illusion!" If Twilight's friends had been there, they'd have told Twilight the look of revelation on Trixie's face was very familiar indeed. "Matter, time, space, they're all nothing more than shadows created by the spiritual state of being. We grow among these shadows, but... we... have to also break free of them."
Trixie then began to glow white and rise up into the air, streaks of blue light dancing and swirling around her, her mane and tail billowing in a sourceless breeze. She slowly rotated in the air as the light show around her became faster and more intense, gradually enveloping the blue unicorn completely. Twilight wondered if Rainbow Dash had slipped Twilight some hard cider. Then with a sudden blinding flash of light, Trixie was gone.
"... Okay... haha Trixie... very funny... nice teleportation, you've truly learned how to teleport for real. I believe you.” Twilight glanced around the balcony, her eye twitching. “You really had me going. You can do real magic. I promise I respect you as a fellow mage. You can come back now." Twilight looked inside the library, then over the edge of the banister, then up at the roof. "I know you're watching. I know you can't resist seeing the audience's reaction, you wouldn't be you if you didn't..." Dead silence. "Come on Trixie, this isn't funny anymore."
When Trixie opened her eyes, found herself in an endless void studded with a million points of light. She was currently standing on a winding road of stardust that stretched to the horizon.
"Where... where is Trixie?" Trixie squinted. "Everything, looks, different somehow."
Princess Celestia appeared. "Hello, Trixie Lulamoon..."
Trixie reacted on a lifetime of ponies judging her. "Whatever Trixie did, she's extremely sorry! Please don't smite the Humble and Contrite Trixie!"
Princess Celestia gave smile number #10 (surprised and so happy for you) and nuzzled Trixie, lifted up her face. "My little pony, it is time."
"This..." Trixie whispered, her eyes slowly widening. "Is no illusion."
Back in reality(?) Twilight was jolted out of her shock when Starlight Glimmer appeared in a flash of teleportation. Starlight looked around frantically.
"My Trixie alarm went off! What did she do this time? I promise she didn't mean it!"
Before Twilight could say anything, a star descended from the sky, and landed gracefully on the balcony of the crystal castle. As they watched, amazed, Trixie's form emerged from the star... now bearing Earth pony muscles, the voice of a merpony, but actually obviously, pegasus wings.
Twilight thought, 'So that's what it looks like from the outside...' "Oh... heheh, okay Luna! You can stop this dream now! I promise to not eat snacks before bed again! ... Luna? Help?"
"Your congratulations on Trixie's ascension to godhood is most humbling," Trixie said flatly. "At least Trixie can trust her best friend to give Trixie the appreciation she has earned from glimpsing the truth of the universe."
Starlight blinked. "And everypony was betting on me being next."
"Trixie is going to go visit Discord now."