Channeling Stallman here, there's no such thing as "intellectual property law". There are a bunch of laws that work in very different ways, evolved under different circumstances, and have different purposes. Copyrights are for ensuring authors retain economic and/or moral rights on their work, patents are so that inventors will disclose their inventions instead of keeping them as trade secrets, and trademarks are so that consumers don't get duped into buying the wrong product. There are other sets of laws that have other purposes and histories, such as regional designation or shipwright designs.
Lumping them all together as "intellectual property" promotes oversimplication and confusion.