I don't think that this is because of functional vs other, but because of the assignment operator and mutable primitives (which functional languages tend to omi

I don't think that this is because of functional vs other, but because of the assignment operator and mutable primitives (which functional languages tend to omit). Consider \x y -> g (f x) (f y) We have no way of knowing what this function will do, because f and g are free variables with respect to the function. But we assume they are constant, so we're happy. And if you zoom out far enough eventually you'll find the binding locations. Instance members and globals are just the imperative version of free variables, and if it weren't for that pesky assignment operator or mutable primitives, they'd be constant and hence completely harmless too. You could imagine an OO language without assignment or mutable primitives that would have all the advantages of "functional" languages but that would still feel different enough in style that we wouldn't call it one.