We pay special attention to the difficulties encountered by scientific knowledge in constructing a classification system that accounts for situations in which the subject of knowledge is mixed or even confused with the object of knowledge. This implies not adequately allocating certain areas of knowledge, which may lie on the border or even belong to more than one scientific field. In order to deepen a discussion on science classification systems, we briefly present two examples. The first was due to Ampère, when he encountered this problem in the preparation of a General Physics course that was being prepared for the College de France. The second system presented was proposed by the Marxist philosopher Henri Lefèbvre, at a time when positivist and scientificist ideas were widely accepted not only in France, but throughout Europe. At the end of the chapter, we propose a possible path for classifying sciences, placing them in an anthropological perspective that breaks the dichotomy between subject and object and works with a system in which both interact and complement each other in the construction of knowledge.