Gratian’s usual method is as follows: He will open with an authoritative proposition. If he finds it universally accepted, it stands as valid. But if there are opposing statements, he tries to reconcile them, either pointing out the difference in date (for the law of the Church may be progressive), or showing that one of the discordant rules had but local or otherwise limited application, or that the first proposition is the rule, while the others make the exceptions. If he still fails to establish concord, he searches to find which rule had been followed in the Roman Church, and accepts that as authoritative. A rule being thus made certain, he proceeds with subdivisions and distinctions, treating them as deductions from the main rule and adjusting the supporting texts. Or he will suppose a controversy (_causa_) and discuss its main and secondary issues. Throughout he accompanies his authoritative matter with his own commentary--commonly cited as the _Dicta Gratiani_.[405] The _Decretum_ was characterized by sagacity of interpretation and reconcilement, by vast learning, and clear ordering of the matter. Only it was uncritical as to the genuineness of its materials; and a number of Gratian’s own statements were subsequently disapproved in papal decretals. The _Dicta Gratiani_ never received such formal sanction by pope or council as the writings of Roman jurists received by being taken into Justinian’s _Digest_. The papal decretals had become the great source of canonical law. Gratian’s work was soon supplemented by various compilations known as _Appendices ad Decretum_ or _Decretales extravagantes_, to wit, those which the _Decretum_ did not contain. These, however, were superseded by the collection, or rather codification, made at the command of the great canonist Gregory IX. and completed in the year 1234. This authoritative work preserved Gratian’s _Decretum_ intact, but suppressed, or abridged and reordered, the decretals contained in subsequent collections. Arranged in five books, it forms the second part of the _Corpus juris canonici_. In 1298 Boniface VIII. promulgated a supplementary book known as the _Sextus_ of Boniface. This with a new collection promulgated under the authority of Clement V. in 1313, called the _Clementinae_, and the _Extravagantes_ of his successor John XXII. and certain other popes, constitute the last portions of the _Corpus juris canonici_.[406]