How do I know what sort of person I am? What is the difference between the two?
I fail to see the philosophical meaning here. In my mind, the only way to answer the question "is the glass half empty or half full?" is with "the glass is both half empty and half full - duh! - why are you asking me such a stupid question?".
(Actually, I prefer the line that "the glass is twice as big as it needs to be".)
[In the next installment of "Nathan's cognitive difficulties", we examine the poorly worded expression "have your cake and eat it too".]