Regulæ Juris, also spelled as Regulae - and - Iuris (Latin for Rules of Law) is a generic term for general rules or principles serving chiefly for the interpretation of canon laws.
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Canonical use
In a specific sense, however, regulæ juris are certain fundamental laws in the form of axioms found in the Corpus Juris Canonici, eleven inserted by Pope Gregory IX at the end of the fifth Book of Decretals, eighty-eight by Pope Boniface VIII in the last title of Liber Sextus Decretalium.
These rules are an exposition of several laws on the same subject, conclusions or deductions, rather than principles of law drawn from constitutions or decisions, and consequently reserved to the last title of the two books mentioned, in imitation of Justinian in the "Digest" (L, l, tit. 17).
While these rules are of great importance, it must be stated that few general statements are without exception. Some of the axioms are applicable in all matters, others are confined to judicial trials, benefices, etc. As examples the following are taken from the Liber Sextus: No one can be held to the impossible (6); Time does not heal what was invalid from the beginning (18); What is not allowed to the defendant, is denied to the plaintiff (32); What one is not permitted to do in his own name, he may not do through another (47).
The Eighty-Eight by Boniface VIII
These 88 legal dicta or maxims known as De Regulis Juris were promulgated in 1298 by Boniface VIII.
Given in Rome by St. Peter's on the Fifth Day of the Nones of March in the Fourth year of our Pontificate—Boniface VIII