If you see the trash can as half full, does that make you an optimist or a pessimist? Since it's half full of garbage, as opposed to daisies or puppies, it's questionable. On the other hand, in the tweeted words of Jill Morris: "Some people look at the glass as half empty. I look at the glass as a weapon. You can never be too safe around pessimists." This is part of a complete episode.
I think of Trash as emptied or not emptied whereas I think of the glass as either full, not full, or partially full, And I think we think of the natural state of trash as needing to be emptied. Therefor, a half full trash can is as non-empty as any other state of the can where it is not empty.
If the can is half full, then the only thing I can be optimistic about is that at some point I need to empty it.
I think of it from the perspective of the self. My function is to fill the garbage can. My function is to empty the glass of water. Therefore, the glass is always half empty, and the garbage can is half full.