Watch this video from the Palsgraf Case: https://www.youtube.com/watch?v=mDEbTudkjhc&t=3s 

The Dissent argues that the line between causation in fact and foreseeable causation is arbitrary. Remember that the railroad employee had no reason to know that the package contained explosives. So who does he owe a duty to? On the other hand, at what point should a person stop being responsible for all of the downstream consequences of his actions?