One rat was placed in a large tub of water with sides so high it could not get out. The tub was placed in a dark room. They timed how long the rat would keep swimming before it gave up. The rat struggled to survive for a little over three minutes, then it gave up.
The researchers then placed another rat in the same tub. This time, however, a bright ray of light was allowed to shine into the room. The second rat swam more than thirty-six hours, seven hundred times longer than the rat with no light.
Why was that? The rat with no light had no hope. He saw only darkness. There was no reason to keep swimming."