A doctor came to a sanitarium to check up on the patients. He sees that everybody is walking around with an empty leash. So he asks a few patients what they are doing.
They all answered that they are taking their dog out on a walk. Only one of the patients said “What are you, blind? I’m holding an empty leash!”
So the doctor says “Very good, I see you are not as crazy as everyone else.”
When the doctor leaves, the patient says to his imaginery dog, “Did you see how we fooled him, Sparky?!”