How can you actually tell if you're awake or dreaming? None of us can.

Clara can't shake the feeling that she's still dreaming. There's still a slight headache lingering in her temple, and she's back in the TARDIS. She theorized out loud that if Santa Claus had truly been her brain's manifestation/interpretation of trying to wake her up—of trying to save her—then the Doctor must have been chosen to fulfill that role to the next level after she vanquished Santa in her dream-state. A dream within a dream within a dream…within a dream.

To what end?

The impossible man, with the impossible box and the impossible adventures.

Of course he couldn't be real. Heroes never actually existed just as you figured they would.

She slammed her hands on the console, crying "How COULD I be so STUPID!" for believing so long. Her eyes widened and she froze at another thought freight-training through her mind. "How long have I been dreaming?" She looked up at the Doctor, who was staring at her, brow furrowed. He responded with silence, his bright eyes boring into her own darkening, haunted ones. There were shadows beneath her eyes that appeared to be manifesting as storm clouds.

"YOU!"

She pointed a finger at him and charged at him, pointing into his chest. "I've known you for ages. Years, even, but dream-time moves faster than real-time, so that doesn't matter. OH my god have I been dreaming all of this? Danny? The kids? The mummy, the moon, the daleks, the cyberman…when did I lose my grip on reality?" She spun on the spot, raising her hands to grip the sides of her head.

The Doctor stepped towards her slowly, but she avoided his route and began to pace around the console, head down and clutched between her hands. "I remember seeing you when I was just a little girl. Did I never have a grip on reality? Were you always there? Of course you were always there—you're a bloody time traveler!" She threw her hands up and glared at the TARDIS column.

"How awfully convenient!"

At some point in her raving, the Doctor had switched directions and seemed to have magically flashed in front of her. He looked down at her and began to speak, a tiredness weighing down each syllable. "Clara. You need to calm down. Just quiet down, and—"

"NO!" She pushed him away and turned on her heel in a whirl. "You're ME saying that so the crab can keep sucking out my brains! I can't trust anything—not even you." She turned, eyes wide in shock as a realization dawned on her that broke her heart. The despair in her eyes even twanged at the Doctor's own old heartstrings. "Especially not you." She raised her hand to her lips as her eyes began to burn with hot tears. The Doctor…the anesthetic…well, who better to give the anesthetic than a proper Doctor? And he's the most proper there is…a doctor of anything and everything.

Her thoughts jumped track with a paralyzing blast and her mouth picked up speed. "Why aren't I dead yet? The dream crab is certainly taking its jolly-good time! I should be dead by now!This is impossible—"

"Clara." His low, calm voice silenced her panic. Somehow, he had gotten close to her again, just as he always seem to do.

"You are the Impossible Girl. Everything real for you is made to be impossible." He reached out, clutching the back of her neck with a firm, warm hand to ground her. "This is real, I am real, and you are most certainly real."

Her voice hit the air in a quiet sob. "But how can you know?"

He raised a thumb and traced her brow, softly caressing the side of her tear-streaked face.

He thought of his seemingly infinite collections of close-calls, and all of the times he should not have survived. He thought of her impossibly perfect leaf that started it all…of Page 1 in this beautiful, beautiful human being's life, of the charm and the smiles despite the pain he knew he'd never fully seen. He thought of how neither should have survived the breach in his time line, or the countless laser blasts they both dodged, or all of the death-traps the universe itself seemed to be made of…How could he know this is real? Because in all of the time he'd seen…it was custom for there to be more blackness than starlight. If you looked closely at the gray areas of everything, you'd find black and white soon enough. Moments and memories and beauty didn't make happiness; life perceives it as such. They are simply there, and life appreciates the neutrality by entrusting a purpose to it, a meaning.

And when he looked at her, the timelessness inside of him already saw her as a memory, and he was sad. Sad before she'd really left…but his disconsolate quality paired polemically with his constantly wavering "achievement" of acceptance.

He knew this was real because it wasn't fair, it wasn't nice

It was just right.

"Because I'm the Doctor. And We're impossible."

The end.