The third canonical period, known as the ius novissimum ("newest law"), stretches from the Council of Trent to the promulgation of the 1917 Code of Canon Law which took legal effect in 1918. The start of the ius novissimum is not universally agreed upon, however. Dr. Edward N. Peters argues that the ius novissimum actually started with the Liber Extra of Gregory IX in 1234. Ius codicis The fourth period of canonical history is that of the present day, initiated by the promulgation of the 1917 Code of Canon Law on 27 May 1917. Benedict XV, in his bull of promulgation, refers to the motu proprio Arduum sane, which was issued by Pius X, March 17, 1904, and gave rise to the 1917 Code. In that memorable pronouncement the late Pontiff stated the reasons which prompted him as the supreme Pastor of souls, who has the care of all the churches, to provide for a new codification of ecclesiastic laws, with a view " to put together with order and clearness all the laws of the Church thus far issued, removing all those that would be recognized as abrogated or obsolete, adapting others to the necessities of the times, and enacting new ones in conformity with the present needs." It is sometimes referred to as the ius codicis ("law of the code") or, in comparison with all law before it, the ius novum ("new law"). From time to time, the Pontifical Council for Legislative Texts issues authentic interpretations regarding the code. The pope occasionally amends the text of the codes. Pio-Benedictine law By the 19th century, the body of canonical legislation included some 10,000 norms. Many of these were difficult to reconcile with one another due to changes in circumstances and practice. The situation impelled Pope Pius X to order the creation of the first Code of Canon Law, a single volume of clearly stated laws. Under the aegis of the Cardinal Pietro Gasparri, the Commission for the Codification of Canon Law was completed under Benedict XV, who promulgated the Code on 27 May 1917, effective on 29 May 1918. The work having been begun by Pius X, it was sometimes called the "Pio-Benedictine Code" but more often the 1917 Code to distinguish it from the later 1983 Code which replaced it. In its preparation, centuries of material was examined, scrutinized for authenticity by leading experts, and harmonized as much