Our third rule about P and Q does not actually tell us that Q cannot be in if there is a P. It tells us that if Q is not in, then P must be in. In other words, at least one of P and Q must be in, or it is not possible that both P and Q are out. This is not to say that one must be in and one must be out- the third rule does not eliminate the possibility that both P and Q are in.
