There may be varying degrees of rights to possession. For example, if you leave a book that belongs to you at a cafe and the waiter picks it up, you have lost possession. When you return to recover the book, even though the waiter has possession, you have a better right to possession and the book should be returned. This example demonstrates the distinction between ownership and possession: throughout the process you have not lost ownership of the book although you have lost possession at some point; or instead, the book may have been owned by a third party (such as a lending library) throughout, despite the changes in possession.
