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xlii PREFACE from another by the addition of a unit. Thus one angel differs from another by the addition of an operation. In addition, a being differs from another being according as in one being there is a reduction of multiple operations to a few unified operations, the being with greater operative unity excelling in measure of perfection the being with less. Any being possesses a constitutive principle by virtue of which it is that which it is and performs that which it performs its proper perfection. Each creature exists by its assigned act and proper perfection. The more an active and operative power is elevated in dignity, the more noble it is, the more power it includes and compounds in itself. Angels are such magnitudes of power completely ac- tualized. "Ordo rerum tails esse invenitur ut ab uno extreme ad alterum non perveniatur nisi per media" 7 The Latin text of the De Ente et Essentia used for this translation is the critical edition of M.-D. Rolland-Gosselin, O. P., LE "DE ENTE ET ESSENTIA" de S. Thomas d'Aquin (Texte etabli d'aprs les manuscrits parisiens.), Bib- liotheque Thomiste VIII, Le Saulchoir, Kain, Belgique, 1926. This edition, which includes an introduction, notes and his- torical comments, is fully authenticated and corrected. Footnotes have been confined to the citation of the text locus when an author or quotation appears in the translation. Refer- ences: Aristotle, any standard text, but preferably in Greek or Latin; Avicenna, Opera... per canonicos (S. Augustini) emen- data, Venise 1508; Averroes, Aristotelis . . . Opera . . . Averrois Cordubensis in ea opera . . . comentarri, Venetiis, in 4, T. I., 1552 et seq.; Boethius, M. S. Boetii Opera Omnia, T. 2. Migne 7 Further concerning the Angelology, cf. Summa Theologica and the Summa Contra Gentes. (English trans, by the Dominican Fathers, Lon- don, Burns, Gates & Washbourne Ltd.; Benzigcr Bros., New York.)