Classification and indexing are great contributions to the foundations of the knowledge organization field. The aims of the presentation are: to highlight the theoretical intersections between some influential authors on classification and indexing of the nineteenth and twentieth centuries (Harris, Dewey, Cutter, Otlet, Kaiser, and Ranganathan) that helped to develop the theoretical-methodological framework of knowledge organization; and to provide a clearer picture of the historical and theoretical contributions to the epistemological foundations of knowledge organization.