Inside a book, the various volumes are not as different from each other as they might seem at first. They are al made up of even more fundamental unit--the word. You can argue that the word is the fundamental building block of the library; and, as was the case for cataloged books, we required a set of rules called grammar, that tells us how to put the words together to make books. Words and grammar, then take you down to a more basic level in your probe of the library's reality. You probably would not be content very long with the notion of the word as the fundamental building block, because all of the thousands of words are different combinations of a small number of more fundamental things--letters. Only 26 letters (at least in the English alphabet) provide the building blocks for all the thousands of words on all the pages in all the books in the library. Furthermore, we need a set of rules (spelling) that tells us how to put the letters together into words. The discovery of letters and spelling would constitute perhaps the ultimate description of a library and its organization.
The library can be described in this way: We use spelling to tell us how to put the letters together into words. Then grammar tells us how to put words together into books. Finally, we use organizing rules to tell us how to put books together into a library, and this, as we shall see, is how scientists attempt to describe the entire physical universe.