All depends what the glass contained in the first place. With some drinks half empty sounds better than half full.
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Chesterton once wrote in this connection: "Upon the whole, I came to the conclusion that the optimist thought everything good except the pessimist, and that the pessimist thought everything bad, except himself." But as a preferred alternative he wrote "An optimist is a man who looks after your eyes, and a pessimist is a man who looks after your feet."