ALTON: Where’s Elaine?
JERRY: Well, we thought she was meeting you earlier. She’s usually pretty punctual. Don’t you find that, George?
GEORGE: Yeah, yeah. She’s punctual... and uh she’s been late, sometimes.
JERRY: Yeah, yeah. Sometimes she’s on time, and... sometimes she’s late.
GEORGE: I guess... (chuckles) today she’s late.
JERRY: It appears that way.
– Seinfeld, S02E03, "The Jacket"
Sometimes you can know something, and sometimes you can't. Sometimes one thing proves that you're in one situation, and not the other. Sometimes that information is relevant, and sometimes it's not.
Sometimes, there are only two possibilities. And sometimes not.
Even in the above example, for instance, there is a third possibility: Elaine might not come at all.