Since air & liquid are elements essential to life on earth.. the answer is both or neither. On planet Earth, the glass is half full of air and half full of liquid, therefore completely full. An answer which is neither optimistic nor pessimistic but scientific.
And then there's the glass or cup that can overflow.. so you have to sip from your saucer.
Just curious, jus asking...
I do hope your glass, mug, cup, canteen, schooner, tumbler, goblet, bottle or vessel of your choice: is good and plenty of whatever you need to be blessed, a blessing and a better person.
xo, me