Jack Sparrow: William, tell me somethin'. Have you come because you need my help to save a certain distressin' damsel? Or... rather damsel in distress? Either one...
Will Turner: No.
Jack Sparrow: Well, then you wouldn't be here, would you? So you can't be here! Q.E.D. - you're not really here!
Pirates of the Caribbean, At World's End
I'm glad IMDB quotes it. Personally when I first saw the movie I laughed a bit too loudly at Jack's logic - and the fact that he said "Q.E.D."
I really am a huge fan of Pirates 1, 2, & 3. I should write about them sometime.
HT to my darling little sister for digging that up for me! :-)