"Is the glass half empty or half full?" is a proverbial phrase, used rhetorically to indicate that a particular situation could be a cause for pessimism (half empty) or optimism (half full), but there are other view points too, like realism (if poured out, half empty; if filled up, half full, but it is only a point of view), or as a litmus test to simply determine an individual's worldview. The purpose of the question is to demonstrate that a situation may be seen in different ways depending on one's point of view.