I have always been curious about what the glass half empty - half full analogy actually means. Usually it's used as an example of optimistic thinking versus pessimistic thinking. Call me strange, but I think that it is actually better used as an analogy of how there are usually two sides to every view, and how what view you agree with is dependant on your perspective.
Many people I have talked to come down on one side or the other of the argument - that is, they assign the positive view to either the glass being half empty or it being half full. There are the types who have actually thought about it on their own and have come to one conclusion or the other. At least these types show evidence of having developed some semblance of their own kinds of thought processes. Then there are others who simply know the "standard" answer (I still can't remember what that is). The interesting thing common to both groups is that, when confronted with an alternate interpretation of the analogy, they cannot understand the thinking behind it. They are so ingrained with their own thought, or worse, someone else's thought, that they can't step outside and catch a breath or fresh air.
This tactic is used, I think, to great effect within the political arena, where people are led to believe a fact or a position is right simply by how a definition or problem is framed. If the first time a person encounters the glass and liquid analogy, they are told that when someone thinks the glass being half full is a good thing because the word "full" is used instead of the word "empty" Then they will find the other explanation hard to grasp. The following is my other definition. When someone says that a glass is only half empty, that is the positive position because half of a bad thing is better than the full of a bad thing (and we all know that empty is a bad thing). So saying that the glass is: only. Half. Empty, means that things are better than they could be. That is positivity in the face of adversity if I ever saw it.
Saying that the glass is half full would mean that it is only half of what would be better, or in other words, not very good at all. That is negative, and yet I can fully allow that the former definition is also accurate, depending on whether you want to focus on the language being used or the facts of the metaphor. This whole issue is a classic example of what I am talking about when I say that: what you see as truth, is dependant upon your perspective or point of view. Change your perspective and you change your opinion. However, I am not promoting relativism. I believe that there is such a thing as ultimate truth and that to arrive at it you must sometimes check your perspective, lest you be deceived.
*My two cents, although I cannot guarantee that all ideas - any ideas - are my own, I just put them together.