I think it's very strong, unmanageable anxiety.
In his case.
In mine: I honestly don't know.
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Yes, we're all replicants!
My friend saw everything in black and white.
He's mentally fine.
Assuming I knew him and not one of his countless doppelgangers! 
I remember his face, though.
I'm going back to my thoughts about inattention and anxiety.
If something resolves itself in part, it's anxiety.
Anxiety is normal.
In excess, it affects the fragile points of different human minds.
Paranoia can affect anyone; if I cross a dangerous area, I'll be paranoid about being in potential danger.
The difference is the context in which it occurs and its pervasiveness.
I lean toward an anxiety-related disorder.
There's a 2006 British study on paranoia as an anxiety-related disorder.
I don't remember the references.
Only the year.