Fig. 1: Possible exception to Theory of Guest Washing Dirty Knife, esp. when sinkful of existing dishes is vastly disproportionate to number of dishes used.
Me: The Dirty Knife Allegory bothers everyone.
Guy Who Related Dirty Knife Allegory to Me (GWRDKAM): What is left to ponder about it?
Me: I don't know, everything. Most people are really adamant that they would wash the knife. I think they're all liars.
Me: No, not really.
GWRDKAM: Why? It doesn't seem so unreasonable.
Me: I think in retrospect it's too easy to say you'd wash the knife. Like, I've thought about it so much that I'm convinced I'd wash the knife.
GWRDKAM: Funny story. The other day I had a drink, and out of habit, I washed the glass, because I always do after using one. And then I thought "oh fuck"!
GWRDKAM: Because I hadn't even thought about it -- and there was a whole sink full of dishes. And so I went ahead and washed all of them, then later my girlfriend [who has since run back into my arms following the original drama of the dirty knife, and all I had to do was take her on a weekend trip to Stockholm!], asks me, "Did you wash all of my dishes?"
Me: THAT'S THE THING. It gets all under your skin.
GWRDKAM: I had totally washed a sink of dishes in a frenzied panic.
Me: Suddenly every sink is booby-trapped.
GWRDKAM: But as my girlfriend pointed out, this is a different scenario. If there's a sinkful of dishes, no one would expect you to wash them. There's apparently a line somewhere between a knife and a sinkful of dishes.
I think I naturally wash only what I use because there's no ambiguity there (in most common situations).
Me: Bare minimum of courtesy.
GWRDKAM: Exactly. Otherwise I'd have to figure out every time what I shouldn't wash and what I should. And I think to wash nothing (not even what you use) would be a bit rude, and standing there for 20 minutes washing all the dishes would be rude, and everything in between is a grey area except washing just what you used.
Me: What if the person catches you in the act and says "Don't worry about it"?
GWRDKAM: Everything in between is a grey area except washing just what you used. [See fig. 1.] That may be the bare minimum of courtesy, but it'd be hard to argue that it was actually rude. And in this world, where all my friends all have different norms and different standards, it seems like the safest bet.*
Me: It is pretty safe.
GWRDKAM: Unless there's just one dish in the sink, I guess.
* This is probably too philosophical for people that are still wondering, "Seriously, what if the host asks you to stop washing the dishes?" I like to think the answer has something to do with whether the person would be receptive to you insisting you're going to finish the dishes ("Oh, I've already started!"), or whether it'd be better to just concede and leave them in the sink. S/he can't really blame you then. Well, possibly.