Horse+cart+paradoxs

During a trip into town a intelligent horse is left on it's own while its owner went shopping. The horse wandered into a local book store and started reading a conceptual physics book. The horse only gets to Newtons three laws of motion before the owner returns. The owner hooks the cart up to the horse and prepares to heads home, only to find the horse unwilling to pull the cart. The horse quotes Newtons third law for his refusal to move saying it would be pointless to apply a force.

Explain why the horse would believe that it would pointless to pull the cart, and explain why it is not.