Assignment and mutability aren't quite the same thing, but they're pretty close. If a.f() has the effect of mutating the internal state of a, how does that occu

Assignment and mutability aren't quite the same thing, but they're pretty close. If a.f() has the effect of mutating the internal state of a, how does that occur? By assignment! In Smalltalk, there's a fairly common pattern of immutable objects. Their state is set when the object is created and then never changed. There might be getter methods, but there are no setters, and in fact the only method that makes assignments to instance variables is the initializer. It's not hard to see how that could be extended from "fairly common" to being a fundamental part of the language.