I don't think this has to be the case! I think you could build a different language with the same message passing semantics as Ruby, but with the limitations that classes are not allowed to be re-opened and method definitions must appear syntactically at the top-level of a class (and maybe a few other limitations), and then build something more similar to how `private` works in a traditional, statically-typed language.

You definitely don't arrive at a language that has all the same feeling of Ruby, but you could still have the "everything is an object, and everything is message passing" character to the language.