You can mutate state on the inside, but still provide a "pure" mathematical function on the outside. For example, a root finding solver. The general problem/feature with objects, is that when a method returns, the object may be in a different state (and often is). Such state changes are hidden but not "encapsulated" as they still contribute to the global state of your application. In other words, the global variables are still there, you've just organised them into modules.