Michael, a very learned orientalist, of the sect of Syrian Christians called Maronites, was born at Tripoli in Syria in the year 1710. As the sect of Maronites were subject to the pope, Casiri came to study at Rome, and entered into holy orders in 1734. In the following year he proceeded to Syria to assist at a synod of the Maronites. He returned to Rome in 1788, and for ten years thereafter taught the brethren of his convent to read Arabic, Syriac, and Chaldaic, giving lectures also in philosophy and theology. In 1748 he passed into Spain, upon the invitation of Ravago, professor to Ferdinand VI., and was by his means employed in the royal library at Madrid. In 1749 he was named a member of the royal academy of history; in 1756 he was appointed interpreter of eastern languages to the king, and soon afterwards joint librarian of the Escorial, with a royal pension of 200 pistoles, besides the ordinary emoluments of the office. In 1763 he became principal librarian, a situation which he appears to have held till his death in 1791.

The only work which entitles Casiri's name to be recorded among the benefactors of literature is his celebrated Catalogue of the Arabic Manuscripts, preserved in the library of which he was keeper. This curious work, entitled Bibliotheca Arabico-Hispana Escurialensis, was published in two volumes folio, at Madrid; the first volume in 1760, and the second ten years thereafter. Mr Gibbon expresses himself "happy in possessing a copy of this splendid and interesting work," which constitutes, indeed, one of the most valuable contributions that modern Europe has yet furnished towards the illustration of eastern literature. The judicious manner in which it is compiled renders it a sort of digest of the attainments of the Saracens in science and literature during the most flourishing eras of their empire. Its contents, as Mr Barington observes, "may, under some of its heads, principally regard Spain; but they will, however, be found adequately to represent the general standard of learning, in its full extent and character, whether at Cordova or Fez, at Cairo or at Bagdad." (Literary History of the Middle Ages, p. 632.)

The manuscripts described amount to above 1800, and are classed in the following order: Grammar, Rhetoric, Poetry, Philology and Miscellanies, Lexicons, Philosophy, Politics, Medicine, Natural History, Jurisprudence, Theology, Geography, and History. The two last classes, with a copious index, occupy the whole of the second volume. As a system of bibliography, this arrangement cannot be allowed to be very perfect or scientific; but it was not the learned author's object to exhibit any such system, though some French bibliographers seem to view his classification in that light, and express no small degree of wonder that an ecclesiastic, in the country of the inquisition, should place so many classes before that of theology, which in their systems generally holds the first place. The title of each manuscript is given in the original Arabic, with a Latin translation, and its age and author, when these are known, are pointed out. But this is not all; the title and description of the work is frequently followed with extracts, also in Arabic and Latin, by which some of the most curious or peculiar features of the piece are brought before the reader, thereby supplying the want of the original, as well to those who could read it if accessible, as to those who could not read it though at hand. The learned author has also collected various interesting and authentic particulars of Saracen biography, and corrected some prevalent errors regarding the lives of these writers.

In the preface, which is of considerable length, Casiri gives a general view of his labours, and commemorates the assistance which he received from the government and from the learned. Rich as the Escorial is in Arabic manuscripts, its present stores are small compared to what they once were; for Casiri mentions that, by a fire which happened in 1670, more than three thousand of these interesting pieces were consumed. They who have not access to this valuable work will find a full view of its contents, with some political comments, in the first appendix to Harris's Philological Inquiries, and in the second appendix to Barington's Literary History of the Middle Ages.