Property law operates on the principle that two people cannot separately own the same thing, but can have conflicting contract rights. This principle applies to all types of property, from land to intellectual property. Understanding this principle helps explain how property law works, including rules about possession, recording systems, and property rights. It also challenges some existing legal ideas, like the importance of exclusion rights. Contrary to popular belief, the complexity of property law is not mainly due to broad duties but rather to the mutual exclusivity principle. This insight suggests that the property law field may need to change its focus.