There's a story I once read:

A philosopher, a priest, and a zen master were sitting around a table, one day. On this table was a glass containing fifty percent water and fifty percent air, roughly. The philosopher and the priest argued long and hard over whether the glass was half full or half empty.

"It's half-full, to show the wonderful bounty our Lord has given us!" cried the priest.
"It's half-empty, to show that we should cast off our complacency and work to better ourselves and the glass!" responded the philosopher.

They argued for some time, until the zen master looked up, took the glass, and prompty drank the water. So it was that they were enlightened.