By the way, did you know that Wikipedia has an entry for "Is the glass half empty or half full?"?
It's a charming example of that the utterly flat, doggedly factual Wikipediprose:
Is the glass half empty or half full? is a common expression, used rhetorically to indicate that a particular situation could be a cause for optimism (half full) or pessimism (half empty); or as a general litmus test to simply determine if an individual is an optimist or a pessimist. The purpose of the question is to demonstrate that the situation may be seen in different ways depending on one's point of view and that there may be opportunity in the situation as well as trouble.There are 8 links in that passage, which, in a concession to the shortness of life, I am not going to insert.
This idiom is used to explain how people perceive on events and objects. Perception is unique to every individual and is simply an interpretation of reality.
There's also a photograph of a half-full glass -- oh! I gave myself away! -- and then this, which I love:
See also"Silver lining," it turns out, has a much richer history than the old 4 ounces of water in an 8 ounce glass, going back to 1634: "Was I deceiv'd, or did a sable cloud/Turn forth her silver lining on the night."