5169. Its posited as an example of quantum superpostition, with the idea that for an unobserved particle, a random subatomic event may or may not have happened. Until the particle is observed, that event remains unknown, and therefore uncertain, so the particle actually exists in both states (i.e. the state in which the event has happened, or the state in which it has not) simultaneously, any only when observation occurs does the superposition collapse into one of the definite states.
The problem for me with using a cat in a box as the analogue, is that regardless of the observer's point of view, the CAT knows if its Alive, Dead or, as Terry Pratchett posited, in the hypothetical state of Bloody Furious.