I do not think that the choice of answers is broad enough to cover all people, or all glasses of liquid for that matter. I do not believe that anyone is either a pessimist or an optimist. There are degrees of each, as well as different ways of looking at a glass of water.
This could be dependent on the relative state of mind of the person you ask. A dehydrated person may answer differently than when no longer thirsty. Ask an optimist about their view of life after an encounter with the IRS or the DMV. Ask a pessimist after winning the lottery. I could be about 51% optimist and 49% pessimist. A few minutes ago it was the other way around.
But comparing personality traits to a partial volume of liquid in an open container has its own problems with physics. You have to be careful with the word ‘empty’ and ‘full’. A non-scientific person may take the term ‘half empty’ as meaning the half without water. Others may believe that empty implies a vacuum, which is impossible with the presence of liquid water. But the liquid in question has not been established as water. They may point out that the state of the clear liquid was not defined. It would be possible to have a glass with a liquid in a solid state in half the container and a vacuum on the other. Most liquids evaporate in a vacuum. To do so within the confines of earth’s gravity there would have to be a lid on the glass. Is a glass of water with a lid still a glass? But let’s assume that it is water. The word ‘full’ may be misleading due to the cohesive properties of water. How can you measure half of something without knowing its total volume? If the glass is half full of liquid water, was the measure of full taken to the rim of the glass or beyond to the point where the cohesive and adhesive properties distort the water? Where do you measure full? It should be rephrased to nearly half full, or statistically half of volume with a percentage of error. Half full or half empty must be absolute. If the glass is a true cylinder then half could be measured with a ruler, but if there is any taper to the glass then a volumetric calculation must take place. This is important because accuracy maters. Half full and half empty are the only two choices given. Maybe it should be rephrased with a margin of error. This could be measured in sigma, for repeatability in further experiments. Another issue is dealing with people who may think out of the box.
I believe that I belong to this group. I do not immediately respond with either half empty or half full. My brain jumps to other things that would explain this situation.
The glass seems to be too large, why would you choose such a large glass for such a small amount of liquid?
The program used to dispense the liquid is not calibrated to the volume of glass that is being used.
This is a waste of materials; it would be more cost effective to adjust the volume to align with the glass size.
How long was that glass left out? It takes time for half the water to change state to a vapour and be lost into the atmosphere. Will the water taste differently due to a larger concentration of total dissolved solids?
Does the glass have a leak? I would have to observe over time to see if there is a reduction in volume. Then it would change the question and have to use smaller fractions. Was the liquid always in this proportion?
Was this test done on someone who was thirsty prior to my arrival?
Part of me also demands that the contents be accurately measured. To me exactly half is very difficult to achieve. An argument could be made that I am a pessimist because I distrust the honesty of the person asking the question. Or I am an optimist because I want more information and agree to continue this verbal jousting.
If I had to commit to the fact that the glass is exactly divided in volume, I would have to say that I see the glass as full of two substances that happen to be in different states (liquid water and air). None of it is empty.
Does that make me an optimist, a realist, or just a geek? Or a jerk? Is there a word for someone who is both a geek and a jerk about it? Sheldon?