Perception.
"But how do we really know, Ryuzaki?" Asked a very frustrated Matsuda. He was obviously sleep deprived and slightly beyond a 'normal' conversation about, say, the weather or who Kira might be.
"Matsuda, are you saying that you're starting to doubt Kira's existence?" L, otherwise known as Ryuzaki, replied.
Matsuda sighed and his shoulders slumped, "No, not exactly. Something out there is killing all these people. Whatever it is. But look, I'm not even talking about Kira! I'm saying that all this," he threw his arms open as if to show he was trying to display the whole world, when in actual fact it was only the whole room, "Everything! How do we know it really exists? At least in the way that we see it, you know, as human beings? All the atoms and molecules moving around and energy and… stuff. It really is beyond comprehension of the human mind! And I really don't think there are such things as 'Universal Truths'."
L stared blankly at him.
"I'm not sure I entirely understand the point you're trying to get across to me, Matsuda."
"Okay, take this table for instance," Matsuda gestured towards the table in front of them. "We might describe it as being square. But how do we really know it is the way that we see it? I'm saying that things might be a lot different from how, we, as physical beings, see things. Ohhh, I can't explain it!"
"Look," said L, "I can't answer those questions. All I can say for certain is that if the table dropped on your head, it would hurt."
Matsuda, without thinking (the unique Matsuda way of doing things), immediately brought his head down to meet the table very hard and fast. He instantly regretted the rash decision.
"Oww…" He moaned quietly with his hands over his head, as if he was trying to make sure it wouldn't fall of his shoulders.
"Were you trying to prove me wrong, Matsuda?"
Through gritted teeth Matsuda persistently managed, "I meant, you know, how the human mind perceives things to be… They may not really be that way on another level of being, or from another perspective."
"I really think you need to get some sleep, Matsuda," Said L, blandly.
Light, completely unaware of the conversation that had just taken place, chose this moment to swivel around in his chair, away from the computer he had been working on.
It took him a moment to take in the scene before him and then gasp, "Ryuzaki! What happened?! Matsuda's forehead is bleeding!"