Well either you have internal state or you don't. If you don't have internal state (and it just returns the answer), then there is no difference between it and a function.
If it does have internal state, then the answer it returns depends on the value of that internal state which means it's hard to verify that the answer returned is correct since you have to know the internal state of the object.
If it does have internal state, then the answer it returns depends on the value of that internal state which means it's hard to verify that the answer returned is correct since you have to know the internal state of the object.