Theory of Glass from Discworld

There are, it had been said, two types of people in the world. There are those who, when presented with a glass that is exactly half full, say: this glass is half full. And then there are those who say: this glass is half empty.

The world belongs, however, to those who can look at the glass and say: "What's up with this glass? Excuse me? Excuse me? This is my glass? I don't think so. My glass was full! And it was a bigger glass!"

And at the other end of the bar the world is full of the other type of person, who has a broken glass, or a glass that has been carelessly knocked over (usually by one of the people calling for a larger glass), or who had no glass at all, because they were at the back of a crowd and had failed to catch the barman's eye.