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The Four Books of Sentences (Libri Quattuor Sententiarum) is a book of theology written by Peter Lombard in the 12th century. It is a systematic compilation of theology, written around 1150; it derives its name from the sententiae or authoritative statements on biblical passages that it gathered together.
The Book of Sentences had its precursor in the glosses (an explanation or interpretation of a text, such as, e.g. the Corpus Iuris Civilis or biblical) by the masters who lectured using Saint Jerome's Latin translation of the Bible (the Vulgate). A gloss might concern syntax or grammar, or it might be on some difficult point of doctrine. These glosses, however, were not continuous, rather being placed between the lines or in the margins of the biblical text itself. Lombard went a step further, collecting texts from various sources (such as Scripture, Augustine of Hippo, and other Church Fathers) and compiling them into one coherent whole. In order to accomplish this, he had to address two tasks: first, that of devising an order for his material, because systematic theology had not yet been constituted as a discipline, and secondly, finding ways to reconcile doctrinal differences among his sources. Peter Abelard's Sic et Non provided crucial inspiration for the latter tasks.
Lombard arranged his material from the Bible and the Church Fathers in four books, then subdivided this material further into chapters. Probably between 1223 and 1227, Alexander of Hales grouped the many chapters of the four books into a smaller number of "distinctions". In this form, the book was widely adopted as a theological textbook in the high and late Middle Ages (the 13th, 14th, and 15th centuries). A commentary on the Sentences was required of every master of theology, and was part of the examination system. At the end of lectures on Lombard's work, a student could apply for bachelor status within the theology faculty.
The importance of the Sentences to medieval theology and philosophy lies to a significant extent in the overall framework they provide to theological and philosophical discussion. All the great scholastic thinkers, such as Aquinas, Ockham, Bonaventure, and Scotus, wrote commentaries on the Sentences. But these works were not exactly commentaries, for the Sentences was really a compilation of sources, and Peter Lombard left many questions open, giving later scholars an opportunity to provide their own answers.