If the encapsulation is implemented in a way that a) it is not possible to instantiate invalid objects of the type and b) all modifications via the type's methods only create valid invariants by enforcing the type's rules (either by mutating it in place or by returning a new instance for every modification), then it is ensured at compile time that this type cannot be misused ("make invalid states unrepresentable"). If that particular logic lives somewhere else, that's not possible.