> In an object-oriented system, each object has its own behavior, and the rest of the program (that is, the environment) has no business knowing how it internal

> In an object-oriented system, each object has its own behavior, and the rest of the program (that is, the environment) has no business knowing how it internally works. I would say that describes Erlang-style message passing, or maybe JavaScript-style prototype OO. But in most "common OO" languages, I would say it's more correct that each class has its own behavior, not object. Of course, the hidden state of an object can parameterize the class's behavior. But still, there are many behaviors which differ based on the type or identity of their arguments, but aren't well suited to be encapsulated into said objects as they're equal peer parameters to the message, hence CLOS etc.