Though Gregory IX. wished to supersede the _compilationes_, he had no
idea of superseding the _Decretum_ of Gratian, still less of codifying
the whole of the canon law. Though his collection is still in theory the
chief monument of ecclesiastical law, it only marked a certain stage and
was before long to receive further additions. The reason for this is
that in most cases the decretals did not formulate any law, but were
merely solutions of particular cases, given as models; to arrive at the
abstract law it was necessary to examine the solution in each case with
regard to the circumstances and thus formulate a rule; this was the work
of the canonists. The "decretalists" commented on the new collection, as
the "decretists" had done for that of Gratian; but the canonists were
not legislators: even the summaries which they placed at the head of the
chapters could not be adduced as legislative texts. The abstract law was
to be found rather in the _Summae_ of the canonists than in the
decretals. Two important results, however, were achieved: on the one
hand, supplementary collections on private authority ceased to be made,
for this Gregory IX. had forbidden; on the other hand, the collections
were no longer indefinitely swelled by the addition of new decisions in
particular cases, those already existing being enough to form a basis
for the codification of the abstract law; and for this reason subsequent
collections contain as a rule only the "constitutions" of popes or
councils, i.e. rules laid down as of general application. Hence arose a
separation, which became more and more marked, between legislation and
jurisprudence. This change was not produced suddenly, the old method
being at first adhered to. In 1245 Innocent IV. sent to the universities
a collection of 45 decretals, with the order that they should be
inserted under their proper titles in the collection of Gregory IX. In
1253 he sent a further list of the first words (_principia_) of the
complementary constitutions and decretals; but the result was
practically _nil_ and the popes gave up this system of successive
additions. It was, however, found expedient to publish a new official
collection. At the instance of the university of Bologna, Boniface
VIII., himself an eminent canonist, had this prepared by a committee of
canonists and published it in 1298. As it came as an addition to the
five books of Gregory IX., it was called the sixth book, the _Liber
Sextus_. It includes the constitutions subsequent to 1234, and notably
the decrees of the two ecumenical councils of Lyons, and is arranged in
books and titles, as above described; the last title, _de regulis
juris_, contains no less than eighty-eight legal axioms, mostly borrowed
from Roman law. The _Liber Sextus_ is cited like the decretals of
Gregory IX., only with the addition of: _in sexto_ (in VI^o.).