Regarding -1 apple, you can argue in the same way that any concept "doesn't exist" if you refuse to recognize a correspondence between it and its referent. What makes "-1 apple" less real than "1 apple"? I could argue, for example, that the conceptual boundary between an apple and the tree it came from cannot be guaranteed to be the same as its actual physical point of separation, and thus any real apple is more like 1.000001 or 0.99999 apples, so there is no such thing as "1 apple" in reality.