An old man went to the doctor complaining of a terrible pain in his leg. 

“I am afraid it’s just old age”, replied the doctor, “there is nothing we can do about it.” 

“That can’t be” fumed the old man, “you don’t know what you are doing.” 

“How can you possibly know I am wrong?” countered the doctor. 

“Well it’s quite obvious,” the old man replied, “my other leg is fine, and it’s the exact same age!”