The glass is full. As the liquid is removed, that space is taken up by air. Context is important,then you have perspective/perception.

That analogy makes sense and I have never thought of it in that light.

I prefer a "safety margin" of greater then 50% buffer in any energy/storage system. A system at or below 50 percent of capacity has excess wasted (unused) capacity and should be replenished so regardless of the fill/deplete vector, the glass is half empty.

It you used pure water and a very clear glass how could you tell, assuming refraction is neutralized,
You would see nothing ergo not know, yet it does exist, only to those that measure the water in relation to the glass

Makes sense to me. Context is everything. Half full / empty is just a snapshot in time. Insufficient data to have a view about its import really.

Yes Quantum physics tells us it is a probability. The glass is a vehicle to be filled with something . If half of it's contents are missing then the glass is not full nor is it empty. To give it a measurement is taking a moment in time and creating a state of matter at that time. That is all that is happening.

So, we’re looking across the valley at a hillside that’s ‘half treed.’ Is it half logged, or half forested? Is the untreed half populated by grazing cattle, or littered with logging slash? Context seems key, to me, too

Actually, yes it does. The context is important. Great point.