Index of Prohibited Books, or simply INDEX, is used in a restricted sense to signify the exact list or catalogue of books, the reading of which is forbidden to Catholics by the highest ecclesiastical authority. This list forms the second and larger part of the codex entitled "Index librorum prohibitorum", which contains the entire ecclesiastical legislation relating to books. The "Index librorum prohibitorum", as an integrant part of the prohibition of books, has already been dealt with in the article Censorship of Books.
A book is prohibited or put on the Index by decree of the Sacred Congregation of the Roman Inquisition, of the Sacred Office, or of the Index, which decree, though approved by the pope (in forma communi), always remains a purely congregational decree. It need scarcely be mentioned that the pope alone, with-out having recourse to any of the congregations, may put a book on the Index, either by issuing a Bull or a Brief, or in any other way he chooses. Formerly it was the rule that a book was examined by one of the Roman Congregations only after complaint had been made to Rome. With regard to the Congregation of the Index, however, Pius X, when reorganizing the Roman Curia by the Constitution "Sapienti consilio (June 29, 1908), decreed as follows: "Henceforth it will be the task of this Sacred Congregation not only to examine carefully the books denounced to it, to prohibit them if necessary, and to grant permission for reading forbidden books, but also to supervise, ex officio, books that are being published, and to pass sentence on such as deserve to be prohibited. Its further task is to remind the bishops of their sacred duty to combat the publication of pernicious writings and give information about them to the Apostolic See, in accordance with the Constitution "Officiorum ac munerum" of January 25, 1897 (Acta S. Sedis, XLI, 432).
In the reorganization of the Roman Congregations, Pius X did not change the constitution or methods of the Congregation of the Index, but rather confirmed anew Leo XIII's Bull "Officiorum", together with Benedict XIV's "Sollicita ac provida" sanctioned therein. This Bull of Benedict XIV, published on July 8, 1753, regulates in detail the procedure of the Roman Congregations in the examination of pernicious books. It strictly commands that the examination of a book be entrusted only to revisors well versed in the particular language and branch of learning. They must be free from all partisanship and prejudice, and must pass judgment not according to their private predilections or the tenets of any school, but simply and solely according to the general Catholic teaching and the dogmas of Holy Church. Especially when examining books of Catholic authors of merit, they ought, in a spirit of fairness and leniency, to allow them free circulation, if at all possible. In no case ought the book of a Catholic author to be condemned on the strength of the verdict of one revisor, not even when all the consultors agree with him. Together with the report of the first revisor—whose name, however, must not be mentioned—the book must be given to another for a second revision, and only when the second revisor's verdict is in agreement with that of the first are both reports referred to the cardinals for final decision. If, however, the second revisor be of opinion that the book ought not to be prohibited, a third shall examine both verdicts as well as the book itself, but without knowing the names of the other revisors. If the opinion of the third coincides with that of the first, and with the general vote of the consultors, the case may be passed on to the cardinals. Otherwise the consultors are again to give their votes, whereupon the matter is put before the cardinals for final decision.
In the case of writings which, according to the decision of the congregation, may be published in a revised edition, the congregation should, if possible, hear the author's own defense or else appoint a consultor ex officio for the defense. If the book have been forbidden with the clause "donec corrigatur" (i.e. until corrected), and the author be willing to publish an edition in keeping with the wishes and orders of the congregation, the decree of prohibition is to be withheld, unless the prohibited edition be already widely circulated and known. In the latter case, when promulgating the decree, the new revised edition is to be expressly mentioned as authorized. The secretary to the Congregation of the Index is empowered to communicate the strictures passed on censured books to the respective authors or their representatives—but to these only at the author's request. Otherwise the official secret is to be strictly observed by all who have taken part in the process. Books, which at first sight are recognized as very dangerously heretical or immoral, may be forthwith prohibited.
The first printed catalogues of forbidden books did not appear at Rome, and, even after the institution of civil censorship, lists of books and writings prohibited by the State continued to appear, and are even yet published (see Hilgers, "Der Index der verbotenen Bucher"). The first Roman "Index of Prohibited Books" (Index librorum prohibitorum), published in. 1559 under Paul IV, was very severe, and was therefore mitigated under that pontiff by decree of the Holy Office of June 14 of the same year. It was only in 1909 that this "Moderatio Indicis librorum prohibitorum" (Mitigation of the Index of Prohibited Books) was rediscovered in "Codex Vaticanus lat. 3958, fol. 74", and was published for the first time in the "Zentralblatt für Bibliothekswesen" (Leipzig, 1909-10). Concerning the curious indexes of 1590 and 1593, which were printed but never promulgated, see Hilgers, "Der Index der verbotenen Bucher", 12 sq., 524 sqq., 529 sqq. The last and best edition of the Index, published by Leo XIII (Rome, 1900) and now in force, was reprinted in 1901, and again under Pius X in 1904 and 1907.