A related question (I think), is what counts as "impossible"? According to quantum mechanics, anything that is not prohibited by a conservation law is possible

A related question (I think), is what counts as "impossible"? According to quantum mechanics, anything that is not prohibited by a conservation law is possible, with some non-zero probability. So we have to pragmatically redefine "impossible" to mean "the probability is so small that it won't happen". But then where do we draw the line between "too small" and "not too small"? (We can also qualify "won't happen" by saying "cannot have happened in the history of the observable universe", and take into account an upper bound on the number of "events" that can ever happen in the history of the observable universe, eg ~ 2^380. So, for example, an event with probability 2^-500 on one occasion has probability < 2^-120 of ever having happened that we could know about.)