At each level of this hierarchy of abstractions one can identify necessary relationships (concepts that mutually presuppose one another), which at succeeding and lower levels incorporate more and more historical and/or geographical variability. This search for the mutually presupposing by separating out the merely contingent is the method identified by Sayer (1984) as rational abstraction. The method can be used to develop concepts at any level of abstraction, including lower level concepts that are better able to incorporate geographical and historical variation.