I think they mean something like abstract base classes. It's tempting to design a class hierarchy from first principles: everything in the game is an entity, f

I think they mean something like abstract base classes. It's tempting to design a class hierarchy from first principles: everything in the game is an entity, from which players and NPCs are derived. The NPC base class has several derived versions (allies, monsters); the monster class is specialized into animals and dragons, which in turn.... You can often paint yourself into a corner this way, where you suddenly don't want the dragon class to reuse part of the MonsterBase. They're suggesting making the classes stand-alone with duplicated code, at least until you figure out what these classes actually have in common and what is incidental overlap.