Look, let's do this mathematically people.
Let G be the set of all possible levels of liquid in a glass, and let g be an element of G such that g is the maximum level of liquid in the glass. Now, let le be the level of water in a glass half-empty, and lf be the level of water in a glass half full. Then,
lf = (1/2)*g, and
le = g - (1/2)*g
But, le = g -(1/2)*g
= (2/2)*g - (1/2)*g
Therefore, le = lf
So, any glass that is half empty is also half full. Now can we please put this question to rest?