Reality
Reality doesn't exist for humans. That's one of the first things he learned in his confinement. You can only view the world from your own mind, and if something is wrong with that mind, it's like looking at the world through a warped lens. But it's not just the image that twists; it's no illusion. If you change your mind, you change the very world in which you live.
So, reality is a moot point. It really doesn't matter what exists outside your mind, because your mind is where you're staying. But, without a concept of reality, doubt grows and festers like mould. And the mind changes, to become an altogether blacker place.
He's not sure that all that happened in the past few years- Sephiroth, the Remnants, Deepground- is real. He could just be dreaming in his coffin, waiting for someone to awaken him. His mind still wrestles with the concepts, in the end falling back upon itself.
The second thing he learned in his coffin was that the mind is always changing. If you put something into the mind, it will change it. These changes will also cause changes, leading to a never-ending cycle.
His body has been taken apart and reconstructed. Hojo has put monsters inside his very soul. That's what he's been told, by records and data. But because he knows this, his world changes. The monsters inside him become more concrete, more real. More threatening. And because he realises that they're more real, he hides himself away even more. His world doubles back on itself. He wonders what exactly is real.
It's a question, a riddle that has no answer. He can smash his mind against it all he likes, but it will still tower above him like a wall of steel.
His fingers ghost across her arms. He wants to reach out and grasp her hand for comfort, but she's still wearing gloves, and he craves the feel of her skin. In fact, she's still clothed, which is somewhat unusual. Her head is resting on his chest, and she burbles in half-snores. The nape of her neck is exposed, and her hair is falling onto his stomach and tickling him. He brushes it away with one hand, and she sleepily mutters something about bread. For a moment, he's envious. He'd give almost anything to be able to dream of simple things like bread and quilt covers and pranks, rather than the complex, spiralling mechanisms of philosophy and reality. But then her fingers twitch, and philosophy suddenly takes a backseat to the tingles dancing across his skin.
And, all at once, he finds the answer to his riddle, thanks to the warm woman in his arms. And he immediately feels stupid, because it's such a universal answer. It's a fearless retort to almost any question and almost any statement. He tests it on his tongue, and asks himself the question in his mind.
"Is this reality, or merely a passing dream?"
"Who cares?"
A/N: Because the greatest revelations are always found in bed. Honestly, I didn't feel like I was able to capture the density of the themes here, but I had to try sometime. Maybe one day, when I've gotten better, I'll give it another shot.