The problem of whether a glass filled to half its possible volume is half full or half empty is poorly stated. The question that shoud come to mind when you hear people babbling about "optimism" and "pessimism" and other such metaphysical gobbledygook is:

Half full? OF WHAT?!

Those who came up with this pseudo-philosophical dilemma must've lived before Pascal's day.* That glass "half full" of water or beer or antifreeze or whatever doesn't exist in a vacuum. Nothing does. There's a gaseous atmosphere surrounding it, and a fundamental property of gases is that they expand to fill the available space. The glass was completely full of before you poured anything into it. The liquid, being a state of "condensed matter," takes up a definite volume. It's going to displace an equivalent volume of gas, and come to rest at the bottom of the glass provided there's a substantial gravitational field present. But the rest of the glass remains full as well.

The glass is always completely full.

But what, you ask, will happen if you take a glass half-full of some liquid and apply a vacuum, pumping the air out? Again, a perfect vacuum is a physical impossibility, and the remaining gas will simply fill all the space available to it, albeit at a lower pressure. And if you keep pumping, sooner or later the pressure inside your glass will drop below the vapor pressure of the liquid that's "half-filling" it. Liquid will evaporate until the system re-attains equilibrium. Visibly, your glass will then be less than half filled (or perhaps more than half-filled if the gaseous phase of your liquid isn't colorless), but it will always remain full of something.

*Pascal, of course, was himself responsible for a good deal of metaphysical gobbledygook. But to the point here, he discovered atmospheric pressure.

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