Overtime
Allen, Brown, and Carr run a shop. At least one of them must always be present to mind the shop, and when Allen leaves he always takes Brown with him.
This means that, if Carr were out, these statements would be true simultaneously:
- If Allen is out then Brown is in (to mind the shop).
- If Allen is out then Brown is out (because Allen always takes Brown with him).
This is a contradiction; they cannot be true together. Therefore, logically, Carr must always be in the shop.
(From Lewis Carroll.)
