[ferd]

How so? Not sure what you mean.

If object A calls a method of object B (composition), then B cannot call back on B, and neither A nor B can override any behavior of the other (And this is the original core tenet of OO: being all about "message-passing").

Of course they can accept and pass other objects/functions are arguments, but that would be explicit and specific, without having to expose the whole state/impl to each other.