Any older history of the Middle Ages likely will have at least a paragraph or two about twelfth-century canon law. It was then that a "scientific" canon law appeared. Shaped by the revived Roman law taught at Bologna, dialectical hermeneutics, and Rome, canon law gradually separated from theology to become a discipline in its own right. The decisive moment was the "publication" of Gratian's Decretum around 1140. Gratian harvested canonical tradition from earlier collections, arranged it in 101 thematic distinctiones and 36 causae (further divided into quaestiones), the latter anticipating the case-study method of legal instruction. Throughout the work, Gratian, assumed to be both a jurisprudent and monk connected with Bologna, added interpretative dicta to resolve seeming contradictions between texts through dialectic. All subsequent medieval canon law was based at least to some extent on this monumental work. 
Since the 1990s, our view of Gratian has been transformed. John Wei's study belongs to a new generation of scholarship shaped by the pioneering work of Anders Winroth (Wei's advisor at Yale). While decades of painstaking manuscript study and close textual analysis already had raised profound concerns about the reliability of the standard edition by Émil Friedberg, published in the late nineteenth century, it was Winroth who demonstrated that there were at least two recensions in the manuscript tradition, a position which Wei accepts. Gratian 1 produced a more compact and organized compilation characterized also by a lack of Roman law. Gratian 2, whether an individual or a team, not only added Roman law but also a considerable body of theological and liturgical texts, notably those contained in a new, third, section to the Decretum, the De consecratione. (More on this below.) While some scholars have argued that an even earlier version is preserved in a St Gallen manuscript--which Wei discusses in detail and rejects (27-33)--all agree that Winroth has forever changed, and advanced, our understanding of the Decretum. 
While most have focussed on Gratian "the jurist," Wei explores "the theologian." For example, against those who have judged "Gratian's use of the Bible to be largely cosmetic and without consequence for his legal thought" (42), Wei argues that "[i]n both theory and practice, the Bible occupies a prominent place in the Decretum" (42). While Gratian I incorporated much patristic commentary on the Bible, Wei demonstrates his use of exegetical works connected to the School of Laon, including the Glossa ordinaria. Gratian I's hermeneutics, however, were not merely derivative. For example, he could identify the Bible with natural law (37), though also recognizing "the need for human intervention in biblical interpretation" (38-39), and generally emphasizing the literal sense. When harmonizing biblical passages with each other or with other authorities, he introduced distinctions "most often between multiple senses of the same word" (55). In sum, "Gratian was a biblical exegete" (65).
Part 2 of Wei's study treats Gratian I's theology of penance. Here, he engages one of the Decretum's most problematic, and disputed, sections, the De penitentia. While some older scholarship had doubted whether this lengthy, third quaestio--further divided into distinctiones--to Causa 33 was original to the Decretum, it is nevertheless a feature of the first recension. Indeed, Wei argues that it was originally a treatise Gratian I composed, and only later was subdivided "by the schools" (101). After treating the history of penitential theology and practice down to the twelfth century, Wei highlights the influence of the Pseudo-Augustinian De vera et falsa penitentia on Gratian I (84-89). Gratian I drew extensively from this treatise which, in contrast with earlier works, highlighted contrition, not "external works of satisfaction" (84). Wei characterizes this approach as "the scholastic turn," a view of penance first identified in the work of Anselm of Laon (89), which provides another point of contact between Gratian I and northern French theology. While older scholarship often maintained some sort of influence of Abelard and Hugh of St. Victor on Gratian it was, in fact, the school--if not the master--of Laon who shaped Gratian's thought (151-153).
Wei devotes much of part 2 of his book to how Gratian I treats two questions of penitential theology: whether God's charity can be lost through sin and if forgiven sins return when one sins again. Wei's close reading of the De penitentia notes its dependence upon not only the De vera et falsa penitentia but also other treatises connected to the School of Laon such as Ut autem hoc evidenter (95, 155-160), and Baptizato homine (151-153). These play a key role not only in source analysis but in Wei's contention that, rather than an afterthought or anomaly in the first recension, the De penitentia was a central concern to Gratian I. He contends that Gratian I composed it as a response to "aspects of the Anselmian theological tradition" (154) and possibly how they had been received in Italy, as evidenced in the anonymous Augustinus in libro vite (161-165).  Gratian I disagrees with Ut autem hoc evidenter, that charity could not be lost (159-160). On the other hand, he could both agree with the author of Baptizato homine that penance could be performed for multiple sins, and not just one (177) but also disagree elsewhere that "punishment due for forgiven sins returns when one later commits another serious sin" (178-179, 183). By engaging with, and sometimes challenging, the new, scholastic theology of penance, Gratian demonstrates that "Italy was not a theological wasteland" (184). However much he drew upon the penitential theology originating in northern France, the compiler of the first recension not only could take a critical stance and, indeed, felt compelled to do so by "contemporary controversies" in Italy (185).
Wei's sixth chapter discusses how Gratian I's interest in penitential theology led him to consider magic and superstition both in the De penitentia itself and in Causa 26; these considered, respectively, "magically induced impotence" and a priest who was an unrepentant soothsayer. While admitting the speculative nature of his argument, Wei contends that Gratian I was not particularly interested in the canon law of magic, whether in regards to marital impotence or divination; rather he chose these topics to "incorporate his theological treatise on penance, De penitentia, into the Decretum..." (190, 222-223). He argues that, while some of the causae appear to treat plausible, if not necessarily always common, legal cases one might encounter in court, many were clearly fictitious, and served as set-pieces for legal argumentation. (Wei even considers the possibility that the "earliest stages of the Decretum consisted solely of causae.") It was for this reason that Gratian I did not insert the De penitentia into Causa 33 but, instead, built the causa around the treatise (194-195, 202-203). He "already knew what he wanted to discuss in the causa" and accordingly constructed a case, which came to be the De penitentia, to "have a reason for examining certain predetermined questions" (206). The same holds true for Causa 26, whose treatment of a priest practicing divination may have served as well as a way for Gratian I to discuss penance (219-220).
Finally, Wei considers Gratian's views, or perhaps better up, lack of views, on sacramental theology and liturgy. Regarding the De consecratione, the third part of the vulgate Decretum, which transmits the overwhelming amount of canons treating the sacraments and liturgy, Wei agrees with the modern consensus that this was a later addition completely unconnected with Gratian I (229-231). Unlike his immediate canonistic sources, for example the Pseudo-Ivonian Panormia and the Italian Polycarpus, Gratian I paid little attention to sacramental theology, though he did consider the question of whether a sacrament performed by a heretic, for example a simoniac priest, was valid (235-236). While drawing upon dicta from Alger of Liège, whose Liber de Misericordia et iustitia was one of his most important sources, Gratian I could not agree with him that heretical sacraments were in every case valid and communicated grace (236-237). Instead, he "sometimes affirms the dogmatic validity of the eucharist, ordination, and other sacraments celebrated by heretics, rejecting only their spiritual efficacy, while at other times he denies both their dogmatic validity and their efficacy" (238).
To Wei, Gratian I was certainly capable of serious reflection on sacramental theology. For example, in Causa 30 he does treat in some detail the spiritual affinities created by baptism (240-244). However, this is an exception. To Wei, the absence of the sacraments in the first recension, for example the question of the real presence in the Eucharist, may well indicate that he found most of the theology, unlike penance, a settled matter (244-246).
The final section of part three considers liturgy. Like sacramental theology, it came later and is largely found in the De consecratione. Here Wei argues that its omission tells us something fundamental about Gratian and his redactors. To Gratian I, the work was intended to be "an analytical concord of discordant canons." There was no reason to consider the liturgy for there was "little conflict" to consider (247, 249). Moreover, Gratian may have judged liturgy more as custom, and thus not a proper subject for legal analysis (290-294). His continuators were dedicated, instead, to creating a comprehensive work, and thus augmented the distinctiones and causae of the first recension with all sorts of texts--not always well integrated--including liturgy. The De consecratione was the capstone (280-281). To illustrate the stark difference in how the first and second recensions treat the liturgy, Wei considers various examples, among them liturgical vestments, consecrated vessels, and the consecration of churches (253-267). The second recension considers these subjects, especially their legal status, far more extensively. Perhaps this indicates, as Wei suggests, the work of "a liturgical lawyer," if not "theologian" (295).
Wei's conclusion is brief, indeed surprisingly so. While mostly repeating earlier assertions, he does admit that the preceding chapters' focus on Gratian as a penitential theologian has made an "unspoken" argument for a "broader approach to scholasticism than historians of canon law have generally taken to date" (299). Instead of rushing to claim Gratian for the jurists, Wei also reminds us that there was still no conception in the middle of the twelfth century of canon law and theology as separate disciplines. Thus, "Gratian the theologian and Gratian the jurist were one and the same" (301).
Wei's extensive bibliography could have also been occasionally augmented, especially in order to aid the non-specialist in learning more about the background or significance of the point in Gratian's text. For example, given the considerable attention Gratian I paid to marriage, more frequent reference to James Brundage's foundational work on the canon law and marriage would have been welcome. Wei's observation that the Bible was less of source of law and more "a starting point and framework" for his legal thinking" (298), a penetrating insight, might have been further supported, and extended, by reference to other scholars who have commented on how the sacred page functioned in twelfth-century canon law.  Finally, when considering whether penance must be done for each and every sin, he considers Gratian I's selection of Nahum 1:9, which states that "God will not judge the same [crime] twice" (132). It is unfortunate that there is no footnote that guides the reader to the scholarship on this very interesting text, which served as one of the sources for the prohibition of double jeopardy, a point of legal practice that has played a prominent role in western legal practice ever since the dispute between Becket and Henry II. 
Wei occasionally admits the speculative nature of some of his arguments. Certainly, there is nothing intrinsically wrong with taking daring views, particularly when they are backed up close reading and support from the sources. This reviewer finds one of his central arguments, the fashioning of Causa 33 around a De penitentia persuasive, if not conclusive. Less certain is the connection of Causa 26 to the De penitentia. Finally, Wei does not hesitate to speculate on, even declare, the intent of his twelfth-century authors. For example, he states that early decretists, for example Rufinus, misunderstood Gratian I's point in Causa 33 (220-223). He may very well be right, but this reviewer is more cautious h343.
To conclude: John Wei has made a fundamental contribution not only to our understanding of Gratian's Decretum but twelfth-century canon law. Any scholar interested in how canon law developed in the "Renaissance of the Twelfth Century" will need to consider Gratian the Theologian. This work will be indispensable for any student of twelfth-century law and theology.
1. Classically expressed by Stephan Kuttner, "The Father of the Science of Medieval Canon Law," The Jurist: Studies in Church Law and Ministry 1 (1941): 2-19.
2. This reviewer wishes to note that the reader should consider, for example, the work of Atria Larson, whose own studies on the De penitentia Wei criticizes, sometimes quite forcefully, at various points, for example on pp. 108-109. Among various works, see her, Master of Penance: Gratian and the Development of Penitential Thought and Law in the Twelfth Century (Studies in Medieval and Early Modern Canon Law, 11; Washington, DC: The Catholic University of America Press, 2014).
3. Wei also provides a critical edition of this work as an appendix.
4. For example, Peter Landau, "Alttestamentlliches Recht in der Compilatio prima und sein Einfluss auf das kanonische Recht," in Studia Gratiana 20 (1976): 113-133.
5. Notably, Richard Fraher, "The Becket Dispute and Two Decretist Traditions: The Bolognese Masters Revisited and Some New Anglo-Norman Texts," Journal of Medieval History 4 (1978): 347-368.