Relations in KOS: is it possible to couple a common nature with different roles?

The purpose of this paper, which increases and deepens what was expressed in a previous work (Mazzocchi et al., 2007), is to scrutinize the underlying assumptions of the types of relations included in thesauri, particularly the genus-species relation. Logicist approaches to information organization, which are still dominant, will be compared with hermeneutically oriented approaches. In the light of these approaches, the nature and features of the relations, and what the notion of a priori could possibly mean with regard to them, are examined, together with the implications for designing and implementing knowledge organizations systems (KOS).