It’s a timeless philosophical question. The answer is based on attitude, and the situation. If we are talking about a drinking glass, to me the answer is quite simple. It depends on whether you’re pouring or drinking. I’ve never heard anyone say, “pour it about half empty.”
So, half full when pouring; half empty when drinking. Of course, if there is any philosopher in you, you might say, “it depends on how thirsty you are and what’s in the glass.”
If it is gallons of that awful stuff you have to drink before having a medical procedure, then it remains half full in your mind until eventually it’s consumed . If you’ve just finished mowing the lawn in the hot sun, your first beer might seem half empty after the first gulp. When it comes to any glass of something to drink, I will stick to my position that “it depends on whether you’re pouring or drinking.” And, I will yield to the notion that it depends on the contents. I have friends, to whom a glass of wine is always half empty.
Now, what about gas tanks? If you are seeing the needle at the half way mark on the dashboard, then it depends on two things. Where you are going, and where you might be going the next morning. If you are on your way home from the supermarket to watch football all day on a Saturday with no travel plans for Sunday, the gas tank remains half full. If you are part way into the Sonoran Desert on your way from Phoenix to Yuma, then it’s half empty.
This question, of course, has no correct answer. That’s why it’s a philosophical question. A more sensible, determinate question and answer would be one like, “What’s the difference between an orange? Answer: A bicycle, because a vest has no sleeves.”