The cover closes more easily than a difficult idea does.

Finishing a book is a physical act and understanding it is not. You can shut it, shelve it, describe it accurately to someone, and still find that a particular argument keeps returning while you are washing up two weeks later. Difficult ideas do not respect the boundaries of the object that delivered them.

When something keeps coming back, treat that as a sign it has more work to do rather than as a failure to move on. Write down the sentence that will not leave. Then let it stay unresolved for a while, since some ideas take a season rather than an evening.