"The glass is half empty" means that you are pessimistic; you think negatively.Пессимистичный “The glass is half full” means that you are optimistic; you think positively. Оптимистичный You “look on the bright side” is another idiomatic way of saying the same thing. Does that help you to understand?

since so many people know this , most of the people will not say that glass is half empty even if they are pessimistic. :)

The glass is always full. 1 half with fluid and one with air. ;)

in layman's defination its right that..glass is half empty or half full, and this depends on our mental state,if we are in positive state then glass is half full and if we are in negitive state then glass is half empty,.but this statement suits only imperfect conditions or things,bcz in perfect conditions and things there whould be no half,(just the matter of understanding) that can you take your mind in positive state or in negitive state or extrem in perfect state-and what is perfect state is another topic :-)