The advantage of being a pessimist is that things can't get any worse than you're expecting, and they might well get a lot better, so there can never be any 'knock backs'.
That doesn't look accurate, jno. I'm sure it is to some degree, but they have equated depression with negative type people, where it usually is the opposite.
If you have a glass and it has, say, water in it to the half way mark. If you were filling it, it's half full, if it was full and you've, say, drunk half of it, it's half empty.