To classify professors' knowledge, the schools of Paris gradually divided into faculties. Professors of the same science were brought into closer contact until the community of rights and interests cemented the union and made them distinct groups. The faculty of medicine seems to have been the last to form. But the four faculties were already formally established by 1254, when the university described in a letter "theology, jurisprudence, medicine, and rational, natural, and moral philosophy". The masters of theology often set the example for the other faculties—e.g., they were the first to adopt an official seal.