Grice in the ladies' room (from LL)
When you're in a stall in a public bathroom and someone rattles the handle of your stall, what do you say to make them go away? There are several options — "Occupied!" or "Just a minute!" are popular, in my door-ratting experience — but when a rattlee, I have always favored the following:
"Somebody's in here!"
An aunt of mine (who apparently uses this same phrasal deterrent) once came back from the ladies' room mulling it over. Why, she wondered, would you say "someone"? You know perfectly well who's in there! It's you! Why wouldn't you say, "I'm in here!"? I giggled all night.
I recently had occasion to say it again, and it suddenly occurred to me it's because of Grice's Maxim of Quantity:
(i) Be as informative as necessary.
(ii) Don't be more informative than necessary
To your interlocutor, rattling away on the other side of the door, it doesn't make any difference whether it's you in particular or somebody else. From her perspective, what matters is whether the stall is occupied or not. Knowing that, it would be pretty self-centered of you to mention that it's you, specifically, who's in there. Really, all you want to communicate is the presence of a warm (and articulate) body in the stall, hence the indefinite.1
1 I keep having the feeling that I've blogged about this before, but a search doesn't turn anything up. Recent posts by Geoff Pullum and the follow-up over on An Individual's Concepts reminded me of the idea. Sorry if it's a repeat!