The example of the glass which is half full or half empty is often used to show how people view the same situation differently.
However, instead of giving equal importance to both the views, we tend to prefer the view that the glass is half full. Those who see the glass as half empty are advised to be more optimistic and see the glass as half full. Nonetheless, the statement that the glass is half empty is equally true.
The half full glass has no more space while the half empty glass has scope to take up more liquid. From this perspective, the half empty view seems to be more positive and optimistic than the half full view.
Yet again, the half full glass of water has the capacity to dissolve salt so it definitely has space within and we cannot say that the space is occupied fully by water. And similarly, the half empty glass is already filled with air so it was never empty, right from the beginning.
Now lets say, we pour out the water and suck out the air from the glass. What is remaining is called vacuum, which is supposed to be empty, really empty. But scientists say that even a vacuum is not empty – it consists of micro particles which are in some sort of flux.
So what does this tell us. That in this universe, there is nothing really empty. The notion of emptiness must always be qualified – empty of what? Like in the case of the half full glass, we can rightly say that it is half empty of water.
Similarly in the Buddhist concept of emptiness, the emptiness is the absence of an enduring self. However, that does not negate phenomena and therefore, emptiness is only in relation to fullness and in itself, it cannot be. Therefore, this is also known as the emptiness of emptiness.