The quote is from MetaTalk via Throwing Things:
My favorite thought-piece about Ferris Bueller is the "Fight Club" theory, in which Ferris Bueller, the person, is just a figment of Cameron's imagination, like Tyler Durden, and Sloane is the girl Cameron secretly loves.Ha ha. Brilliant. (I've never seen a Metafilter comment with that many "favorite" votes — 381, including mine.)
One day while he's lying sick in bed, Cameron lets "Ferris" steal his father's car and take the day off, and as Cameron wanders around the city, all of his interactions with Ferris and Sloane, and all the impossible hijinks, are all just played out in his head. This is part of the reason why the "three" characters can see so much of Chicago in less than one day -- Cameron is alone, just imagining it all.
It isn't until he destroys the front of the car in a fugue state does he finally get a grip and decide to confront his father, after which he imagines a final, impossible escape for Ferris and a storybook happy ending for Sloane ("He's gonna marry me!"), the girl that Cameron knows he can never have.
I wonder what other movies are susceptible to the "Fight Club" theory.
AND: For law fans, here's the AskMetafilter thread aimed at listing all the crimes Ferris Bueller and his friends commit during "Ferris Bueller's Day Off."