Try this: a librarian wants to create a catalog listing all the books in the library. What about the catalog itself? List it? Then we need a new catalog with the catalog on the list and so forth or need to create a catalog of catalogs that do not contain themselves, or some darn thing. This is called Russell’s Paradox and Bertrand Russell published it in The Principals of Mathematics (1903). It is an area of applied mathematics called naive set theory. I remember reading that Russell struggled with this and it may have broken his mind, so don’t try this at home.