Is the glass half empty or half full?

Is the glass half empty or half full? is an idiom.  

The question is a common expression with both an obvious meaning and a meaning which is inferred or implied.Harley, Trevor. (2010).  Talking the Talk: Language, Psychology and Science, p. 212.

The phrase describes two different ways of understanding the meaning of a situation.Heacock, Paul. (2003). 今天「「今天上午 Cambridge Dictionary of American Idioms, p. 148.

The idiom is used rhetorically to indicate that a particular situation could be a cause for optimism (half full) or pessimism (half empty).Ammer, Chritine. (1997).  The American Heritage Dictionary of Idioms, p. 252.

Metaphor
The phrase is a metaphor. For example, 
According to musician Jelly Roll Morton, "If a glass of water is full, you can't add any more water, but if you have half a glass, you can always put more water in it -- and jazz music is based on the same principles."Sales, Grover. (1992).  Jazz: America's Classical Music, p. 57.

Related pages
Blind men and an elephant

 References 

Category:Idioms
Category:Metaphors