Two-actor theory
This is my coinage for the next step up from solopsism. Where the solopsist sees only one person in the universe, the two-actor theorist sees two: himself, and everybody else. It expresses itself in logic like "I can be mean to you because someone was mean to me this morning.
I rather suspect that modern urban living nourishes this sort of thinking. Our emotiional reactions aren't built for so many random anonymous encounters, so we cram "other person" into one team, to be handled collectively.
I find this concept useful enough that I suspect someone else must have coined a less-ungainly phrase for it already.
comment by Aaron:
"Paranoia" is not so far off. If the rest of the world, collectively, has the unity of intent that most individual people do, the most reasonable reaction probably is to sit around pondering how it all fits together. The postman must have given information about me to the guy who was staring at me on the train, etc.
Are there actually any solipsists out there who think only they exist, or just philosophers who don't think anybody else's existence can be proven?