The British Museum requires other libraries to provide it with a catalogue of their books. Some of these catalogues include an entry on themselves and some do not.
One day a BM librarian had the brilliant idea of classifying the catalogues into those which include a reference to themselves and those which do not in order to avoid confusion (of a form peculiar to librarians)
Later on the same librarian thought that it would be handy to have a “super” catalogue of the catalogues in the two sections created (catalogues with references to themselves and catalogues without such a reference). That seemed like a pretty good idea until the following question was posed:
Should the catalogue of all catalogues which don’t include themselves include itself or not?
There are only two possible answers to this question: “yes” or “no”. So the librarian investigated the consequences of both.
If we answer “yes” then the super-catalogue of catalogues which don’t include themselves includes itself and therefore has a reference to a catalogue which includes itself. This is incorrect since it is only supposed to include a list of catalogues which do not include themselves.
If we answer “no” then the super-catalogue of catalogues which do not include themselves is incomplete since it does not include itself even though it is a catalogue which does not include itself.
Thus, whichever answer is given, the catalogue does not do what it was supposed to do.
Faced with a dilemma from which there was no solution there was only one logical outcome: the librarian committed suicide.
After everyone had had an opportunity to pay their respects, it was agreed that, out of respect for the terrible circumstances which lead to the librarian’s premature exit from this vale of tears, it would be appropriate to inter the body in a paradoxical section of the cemetery reserved for barbers who only shave people who do not shave themselves.