> What guarantees that when I send a message to an object, that object won't decide to change itself in response? I don't think that's what the other commenter

> What guarantees that when I send a message to an object, that object won't decide to change itself in response? I don't think that's what the other commenter is saying. They're saying that internal state is not directly manipulable from external sources. So an object `x` cannot have a method that takes in another object `y` and does `y.foo += 3`. Instead, it must do something like `y.addFoo(3)` to accomplish the goal. This is superficial in some sense, but then every distinction between various programming languages is superficial if you look at it right. :) The idea is that the object itself is the only thing which can manipulate its internal state. Therefore, to use these objects, the programmer must have imbued them with the necessary methods. This is restrictive (you have to implement the methods before you can manipulate the state), but this is the core design principle I think the other commenter is trying to tell you about. Objects cannot be manipulated willy-nilly; they must have implemented some method which can be called from the outside. This restriction creates a much more solid barrier of distinction between the responsibilities of the various objects at play.