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.
This idiom is used to explain how people perceive events and objects. Perception is unique to every individual and is simply one's interpretation of reality. The phrase "Is the glass half empty or half full" can be referred to as a philosophical question.