> The idea is that when you are, for example, writing business logic with books, then you want to neatly put the operations on the book near the definition of t

> The idea is that when you are, for example, writing business logic with books, then you want to neatly put the operations on the book near the definition of the book. That's not the idea. You put the code for the "internals" of a book near to its definition. In other words: The code that needs to see the private/protected members of a class. It is not good OOP - if I may be so bold as to use that phrase - to stick as much functionality as you can involving some class, into that class.