Ibn Sina wrote many works in Arabic and Persian, and a number of these are still extant. Amongst his productions were _as-Shafa_, an encyclopædia of physics, metaphysics, and mathematics in eighteen volumes (ed. Forget, Leiden, 1892), a treatise on logic and philosophy, and the medical works on which his fame so largely rests. The best known of these are the _Najat_ abridged from the _as-Shafa_, and the medical _Canon_, in which he reproduced the teaching of Galen and Hippocrates with illustrative material from the later medical writers. The _Canon_ is more methodical in its arrangement than the _al-Hawi_ of Razes, hitherto the popular manual of medicine in Arabic; indeed, its chief defect is an excessively elaborate classification. It became the leading medical authority, and, after translation into Latin by Gerard of Cremona, served for many centuries as the chief representative of the Arabic school of medicine in western Europe, holding its place in the universities of Montpelier and Louvain down to A.D. 1650. Ibn Sina treats logic as of use rather in a negative than in a positive way: “the end of logic is to give a man a standard rule, by observing which he is preserved from error in reasoning” (_Isharat_ ed. Forget, p. 2). His treatise on this subject in _Tis´ Rasa´il fi-l-Hikma wa-l-Tabi´yat_ (p. 79, pub. Stamboul, 1298), is divided into nine parts corresponding to the Arabic canon of Aristotle, which includes the _Isagoge_ as well as the Rhetoric and Poetics. He makes special note to the logical bearing of particular grammatical constructions which in Arabic differ from the forms used in Greek, as, for example, where the Greek expresses the universal negative by “all A is not B,” but Arabic renders this “nothing of A (is) B.” He lays great emphasis upon accurate definition, which he describes as the essential basis of all sound reasoning, and to this he devotes much attention. Definition proper must state the _quiddity_ of a thing, its genus, differentia, and all its essential characteristics, and is thus distinct from mere description, which need only give the propria and accidents in such a way that the thing may be recognised correctly.