On the necessarily non-empirical nature of political philosophy (or why political philosophy is not a sub-discipline of political science)

Is political philosophy a part or a sub-discipline of political science? In this paper I defend the claim that, although there is substantial overlap between the two, political philosophy is not a mere subset of political science, since it has distinct questions upon which it is focused as well as methods that are often remarkably distinct from those of political science. Those questions are immune to resolution by empirical methods, Accordingly, I political philosophy is not a part of political science because it deals with questions that cannot be resolved entirely using empirical methods. It is, in this sense, ‘speculative’ to a degree that political science is not.