Subject description is a linguistic practice, and therefore all controlled vocabularies are built upon theories of language. However, the underlying theories are rarely discussed in greater length. The purpose of this thesis is to investigate what epistemological foundations controlled vocabularies are built upon and what consequences that has for their internal structure. The main object of interest is the category. From a rationalistic point of view categories are universal and well defined. In a pragmatic point of view categories are contextual with fluid boundaries. A pragmatic point of view has been shown by Wittgenstein and Rosch to more accurately describe how language works, therefore the standpoint of this thesis is that a controll...