What does "internally and externally contested"mean?

Context:

The study of international relations takes a wide range of theoretical approaches. Some emerge from within the discipline itself; others have been imported, in whole or in part, from disciplines such as economics or sociology. Indeed, few social scientific theories have not been applied to the study of relations amongst nations. Many theories of international relations are internally and externally contested, and few scholars believe only in one or another. In spite of this diversity, several major schools of thought are discernable, differentiated principally by the variables they emphasize—eg military power, material interests, or ideological beliefs.