there is someone in the pub such that if he/she is drinking everyone in the pub is drinking.
proof:
The proof begins by recognizing that it is true that either everyone one is drinking - in this particular round of drinks - or at least one person in the pub isn't drinking.
On the one hand, suppose everyone is drinking. for any particular person, it can't be wrong to say that if that particular person is drinking, then everyone in the pub is drinking - because everyone is drinking.
Suppose, on the other hand, that at least one person isn't drinking. for that particular person, it still can't be wrong to say that if that particular person is drinking, then everyone in the pub is drinking - because that person is, in fact, not drinking.
either way, there is someone in the pub such that if he or she is drinking, then everyone in the pub is drinking. hence the paradox