  Despite the fact that limiting relations between theories may be singular in this way, it is (at times) useful and appropriate to think of physical theories as forming a hierarchy related by length or energy scales. The idea is that different theories may apply at different length or energy scales. If one takes this idea seriously, then it may very well be the case that each theory in this hierarchy will be phenomenological relative to those theories at higher energies or shorter distances. Equivalently, such a hierarchy may form a tower of effective theories. An effective theory is one that describes the relevant phenomena in a circumscribed domain—a domain characterized by a range of energies, for example.