The man sat quietly in a corner, thinking. That is, he believed tentatively that the walls he leaned against existed, and that they supported a functioning biological contraption of skin, muscle, tendon, bone, and so forth. The tissue-thin faith that let him believe in his own existence did not extend past what he could feel. "I think, therefore I am" held no special magic.
Being watched helped him believe. He looked through the bars at the girl. Did he believe in the inklings of lust that he felt at the sight of her chest rising and falling with each breath? Did he care that she noticed where he was staring? It might be. There was a hint of embarrassment underlying the anomie.
But maybe she didn't exist. Maybe there weren't any bars. All he could feel was the corner. It could be only the corner, and him. And the rest was his mind trying to distract him. Or his imagination letting him have a pleasant fantasy. About a girl who watched him and breathed, her round breasts moving in a slight, enticing way as he imagined her inhaling and exhaling.
Or maybe even the corner wasn't there. Could the corner be an illusion of touch and support, when really there was nothing but tiny atoms whirling in a void? Even the atoms could be make-believe. He wasn't there at all, he thought.
He decided that the girl was real, but he was not. He was her imaginary companion rather than she being his. She was bored, so she thought up a man to ogle her. She might not even really be attractive, she just imagined him as a man who would think her so.
No, the girl wasn't real either. None of them were. Nothing was... nothing was real... even nothing wasn't real. Reality was a non-event. It was something that never happened. And so everything was imaginary, even imagination. He found this thought satisfying. And so no more thought was necessary.
There never was a man sitting quietly in the corner, thinking.