"Horse Sense" joke

An out-of-towner accidentally drives his car into a deep ditch on the side of a country road. Luckily a farmer happened by with his big old horse named Benny.
The man asked for help. The farmer said Benny could pull his car out. So he backed Benny up and hitched Benny to the man's car bumper.
Then he yelled, "Pull, Nellie, pull."
Benny didn't move.
Then he yelled, "Come on, pull Ranger."
Still, Benny didn't move.
Then he yelled really loud, "Now pull, Fred, pull hard."
Benny just stood.
Then the farmer nonchalantly said,
"Okay, Benny, pull."
Benny pulled the car out of the ditch.
The man was very appreciative but curious. He asked the farmer why he called his horse by the wrong name three times.
The farmer said, "Oh, Benny is blind, and if he thought he was the only one pulling he wouldn't even try."