Hans the horse did NOT know how to add, subtract, multiply or read but it was not discovered for a long time. It wasn’t until a psychology student came along and realized that von Osten was sending the horse subconscious signals that he didn’t even know he was giving the horse. Why wasn’t this discovered for such a long time?

Humans want to be entertained and are willing to believe the unbelievable.
No one smart enough had seen them yet.
The horse did know how to add.
von Osten was incredibly intelligent and knew how to hide the signals he was sending the horse.
NEED