It's hilarious watching people struggle with this. The whole point is that you have to think about it - it depends on philosophy.

In other words, it's all in your head. A real, physical, "negative apple" does not exist in this world. Only the human philosophical concept of "owing someone apples" or similar such things. That's the guys' point. It doesn't matter whether you can bring representations of negative apples because those all depend on a philosophical bedrock.