Why can't I create an enum in an inner class in Java?

What I try to do is this:

public class History {
    public class State {
        public enum StateType {

Eclipse gives me this compile error on StateType: The member enum StateType must be defined inside a static member type.

The error disappears when I make the State class static. I could make State static, but I don't understand why I cannot declare an enum in an inner class.