I think this means that if we look hard enough, we can always see things as more full than empty.
What kind of person do you prefer being around?
When people walk away from you, do they feel more full because of you?
Empty and full are not absolutes - they are perceptions, which means we get to choose.
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I challenge you to SEE FULL this week. When you catch yourself seeing the empty glass, QUICK - shut your mouth - don't say a word - pause and use your brain to SEE the full glass. My bet is that you feel more energized - less drained - and so will everyone that comes in contact with you.