Classification
A 'ranking system' is an organized and hierarchical system of categorizing objects. Numerous domains rank according to the objects to be categorized: living species, diseases, products or services, stars, library documents, etc. Rankings in a limited domain are generally well accepted by specialists in the field. Universal classifications can not disregard a point of view and are therefore the subject of much criticism. They do, however, shed light on the nature of knowledge. The rankings are essential tools to organize the knowledge and the work of each one within the whole. To classify objects or knowledge is to say how they are situated in relation to each other. Several complementary points of view can be considered. The classifications are distinguished according to the formalism brought either to the classes, or to the relations between these classes and to the resulting tree.