Talking to yourself is a sign of impending mental collapse.

More from The Hitchhiker's Guide to the Galaxy

• “All right,” said Ford, “I’ll try to explain. How long have we known each other?” “How long?” Arthur thought. “Er, about five years, maybe six,” he said. “Most of it seemed to make some sense at the time.”
• And no sneaky knocking down Mr. Dent's house whilst he's away, alright?
• “Are you telling me,” he said, “that you set yourself up to become President of the Galaxy just to steal that ship?” “That’s it,” said Zaphod with the sort of grin that would get most people locked away in a room with soft walls.
• Arthur glanced around him once more, and then down at himself, at the sweaty disheveled clothes he had been lying in the mud in on Thursday morning. “I seem to be having tremendous difficulty with my lifestyle,” he muttered to himself.
• Arthur stared into his beer. "Did I do anything wrong today," he said, "or has the world always been like this and I've been too wrapped up in myself to notice?"
• As soon as Mr. Prosser realized that he was substantially the loser after all, it was as if a weight lifted itself off his shoulders: this was more like the world as he knew it.