Speaking in high-falut'n scientific terms..
I propose that if the state of the glass prior to the one is question is known, we can then determin the present state ie.
if we know it the volume was greater than it is now we may say it is half empty. Further, if we know that the volume was lower before it came to it's present state we may state that it is half full.
If we have come upon the glass upon the state in question and do not know how it came to be that way, it must be regarded as being both half full and half empty at the same time. Meaning either answer is half right!
That's my story and I'm sticking to it!