Is the glass half full or half empty?
The pessimist sees what's present today and can only imagine eventual decline. The glass is already half empty and it's only going to get worse.
The optimist understands that there's a difference between today and tomorrow. The glass is half full, with room for more. The vision is based on possibility, the future tense, not the present one.
Pessimists have trouble making room for possibility, and thus possibility has trouble finding room for pessimists.
As soon as we realize that there is a difference between right now and what might happen next, we can move ourselves to the posture of possibility, to the self-fulfilling engine of optimism.