An optimist may argue that his/her glass is half-full. Rather than looking at what is missing, they look at what is left behind. There is still half a glass of water left to enjoy! They take the same attitude towards life in general: it is not what is missing that is important but what is left behind. They recognize what is gone, but instead of dwelling upon that focus on what is there. To them, there is always something good to be found in a given situation.
A pessimist may look at the same glass and say it is half-empty. They are looking at the half of the water already consumed. They are looking not at what they have, but what has been lost. Again, this attitude can extend to daily life. A pessimist may look not at what they have or at the good points of a situation, but at what has been lost. They may recognize the good points, but what is missing may loom larger to them.
I look at the same glass and say that it’s simply half. It is both half-full and half-empty. After all, when you drink half a glass of water, the amount of water in the glass is indeed halved but the amount of space available for air to occupy increases by that much. It is half-full of air and half-empty of water. Or, if you prefer, half-full of water and half-empty of air.
The optimist and the pessimist are both right. Each one is just missing half.