Paul Marie Ghislain Otlet (/ɒtˈleɪ/; French: [ɔtle]; 23 August 1868 – 10 December 1944) was a Belgian author, entrepreneur, lawyer and peace activist; he is one of several people who have been considered the father of information science, a field he called "documentation". Otlet created the Universal Decimal Classification, which would later become a faceted classification. Otlet was responsible for the development of an early information retrieval tool, the "Repertoire Bibliographique Universel" (RBU) which utilized 3x5 inch index cards, used commonly in library catalogs around the world (now largely displaced by the advent of the online public access catalog (OPAC)). Otlet wrote numerous essays on how to collect and organize the world's knowledge, culminating in two books, the Traité de