People who see a half-filled glass as either a) half-empty or b) half-full tend to be either pessimists or optimists respectively. Your writer is trying to say that looking at the situation from a different perspective gives a different range of possibilities--in this case that the glass can be made completely full again. so the half/half question falls away!

People can look at a glass with some water and think two things. 1-It's half full 2-It's half empty These two ways of looking at the same thing has provoked a lot of psychological discussion about a person's personality. The writer is saying that we need to stop arguing whether it is half full or half empty. because what does it matter, when you can refill the glass easily. The last sentence states that the writer feels that point people need to focus on, is the fact that they can refill it.