Al-Razi compiles the Kitāb al-Ḥāwī (Comprehensive Book), a 23-volume medical encyclopedia integrating Greek, Syriac, Indian, and Persian medicine with his own clinical observations. It contains the first documented differential description of smallpox and measles as distinct diseases, based on systematically observed symptoms. He introduces cross-verification of sources — always citing his authorities — and repeated clinical observation as a diagnostic criterion. Translated into Latin in 1279 as Liber Continens, it was a reference text at European universities for three centuries. Al-Razi also directed the first documented psychiatric hospital in Baghdad.