If the last thing that was done with the glass added liquid (filling in a bit, etc), then it's half full.
However, if the last thing that was done took away liquid (for example taking a sip, etc) it's half empty (or halfway to becoming empty if you wanna nitpic )
that's the only thing that really makes sense to me.
In my first post, I said it was both, and when you look at the circumference of life, this is in fact true. Perception is a key tool to have within one's personal machine.
Some of us can be a optimist dependent upon the event, situation, or circumstance.
Some of us can be a pessimist dependent upon the event, situation, or circumstance.
The half full or half empty question sometimes demonstrates how one can view circumstances, events, and situations, in different ways--dependent on the point of view.
my answer is - there IS enough! in there, there is enough glass, and there is enough room for more, and there is enough space filled. It is simply this