 EH is often presented as a hierarchy of increasing degrees of randomness in deterministic systems: the higher up in this hierarchy a system is placed the more random its  behaviour.[20]  However, the definitions of different levels of EH do not make explicit appeal to randomness; nor does the usual way of presenting EH involve a specification of the notion of randomness that is supposed to underlie the hierarchy. So there is a question about what notion of randomness underlies EH and in what sense exactly EH is a hierarchy of random behaviour.