The twelfth-century treatise On Diverse Arts by someone who called himself Theophilus has been well known in the modern world since the eighteenth century, when it was first published by none other than Gotthold Ephraim Lessing, the playwright, art theorist, and sometime librarian at Wolfenbüttel, where one of the important manuscripts of the text is housed. (Lessing was particularly interested in Theophilus's medieval text for its mention of paint that used oil as a binder, since this pushed the historical origins of oil painting back centuries before the van Eycks, that medium's traditional inventors.) Theophilus's treatise is of clear importance to the study of Romanesque art, especially since it may well have been written by the goldsmith Roger of Helmarshausen, the rare medieval artist to whom extant works of the highest quality can be assigned with something close to certainty. This would make On Diverse Arts a unicum: a text on art authored by an important medieval artist.
Despite all this in its favor, art historians have not been entirely sure what to do with On Diverse Arts. In large part, this is because its genre has been difficult to grasp. Is it a practical book of recipes (these form the bulk of the text) or is it a theoretical treatise on the value of art (an interpretation that gives great weight to the section prologues)? Few previous treatments of Theophilus have attended equally to both aspects. Heidi C. Gearhart, in her fine study, has recognized that identifying the genre of Theophilus's text is crucial to its interpretation. Even more important, she saw that a way to make that identification was to use the various manuscripts of On Diverse Arts as a guide to reading. These manuscripts are not Gearhart's discovery; all are well known and have been extensively studied, but with the goals of establishing a correct text and identifying different textual recensions. Gearhart has used the manuscripts for a different reason: to see what information they provide about how Theophilus's text was read in the Middle Ages. Gearhart, employing an approach associated with the new medieval philology, takes the contents of manuscripts in which On Diverse Arts is found to show how medieval readers, writers, and librarians understood its genre and therefore much of its meaning. Each of her book's four main chapters is organized around one of the medieval manuscripts of Theophilus's text. This is a clever rhetorical device; even more important, it is a convincing hermeneutical one.
By paying attention to the manuscript contexts of On Diverse Arts, Gearhart has been able to show that Theophilus's text is "far more learned" (7) than one would expect of a craftsman's handbook. She convincingly argues that the treatise is not (or is not just) a recipe collection; the text is not sufficiently detailed to function as a practical handbook and lacks the illustrations that a primarily practical guide would require. She also rejects the extreme opposite reading, which sees On Diverse Arts primarily as engaged in a theological defense of art, part of the twelfth-century contest that pitted Suger of St. Denis against Bernard of Clairvaux. (While the primary context for many of the manuscripts of Theophilus is Benedictine monasticism, and this point is crucial to much of Gearhart's analysis, she also notes that there is also at least one Cistercian manuscript of On Diverse Arts.) Instead of seeing Theophilus's treatise as a craftsman's handbook or part of the Christian polemic about the status of art, Gearhart reads it as evidence for "a distinctly medieval theory of art" (1), one that gave a sophisticated account of making and materials by viewing them through the lenses of Christian theology and moral philosophy.
Gearhart's study lays out four key aspects of On Diverse Arts. Her first chapter, "Pedagogy and Exegesis," is centered around the famous Wolfenbüttel manuscript, which also contains Vitruvius's classical treatise on architecture. This is clearly high company and by itself puts the lie to the notion that Theophilus's text is only a craftsman's handbook; as Gearhart puts it, this context indicates that "On Diverse Arts was not just a haphazard collection of recipes but part of a scholarly tradition" (17). A touchstone for her analysis in this chapter and throughout much of the book is Augustinian philosophy and theology She is convincing about Theophilus's Augustinianism, which was perhaps transmitted to him via contemporaries such as Hugh of St. Victor; her demonstration of the Augustinian nature of On Diverse Arts is further evidence for the high intellectual status of Theophilus's text.
Chapter 2, "The Transformation of Matter," works out from a thirteenth-century manuscript of On Diverse Arts at Cambridge, where Theophilus appears alongside Palladius's late antique treatise on agriculture and a medieval text on the properties of herbs. Gearhart uses this context to show how matter and the human working of it are central to Theophilus (and, of course, also to much of Christian theology). This chapter is one of the strongest in the book, in part because it serves to historicize a concern that undergirds much art-historical (and other) thought these days. When Gearhart in this chapter's conclusion writes that, for Theophilus, "Metal...is not static, but it is instead dynamic physical matter" (66), she lays the ground for an argument that a currently influential book, Jane Bennett's Vibrant Matter (2010), has a significant medieval historical antecedent. Gearhart, however, does not cite Bennett and, in general, her book hews closely to the twelfth century without explicitly drawing connections to the contemporary world; for many readers, this will be one of the virtues of Theophilus and the Theory and Practice of Medieval Art.
Gearhart's third chapter is devoted to Theophilus's ideas about another crucial Christian concept: work. "Physical activity, then, was not separated from moral activity, but integral to it" (82). This point is basic, but Gearhart makes it with considerable subtlety. For example, she notes that On Diverse Arts praises both zeal and care, but distinguishes between them. This leads her to wonder, when looking at the Enger cross of Roger of Helmarshausen (Berlin) and its intricate filigree work, whether, when viewed through Theophilus's eyes, the entire cross might manifest Christian zeal, while details such as the filigree could be understood to embody care. While not all of Gearhart's readers will buy this reading, it is interesting and serves to tie Theophilus's text to medieval objects contemporary to it. The same is true of her related interpretation of the functionally excessive number of studs on the portable altar of Henry of Werl in Paderborn as emphasizing labor, to which Theophilus ascribes high moral value. Making such associations between medieval writing about art and medieval objects and images has always been difficult (and here one thinks of Lessing, Theophilus's first editor, whose Laocoön  argued that there was a fundamental incommensurability between word and image), so Gearhart's willingness to attempt to bring treatise and metalwork into accord deserves praise.
The significant attention to twelfth-century objects in chapter 3 points the way to Gearhart's fourth chapter, on technique. This chapter draws its inspiration from the Vienna manuscript of On Diverse Arts, surprisingly (and tellingly) the only Theophilus manuscript to contain decoration. This is the manuscript that gives Theophilus's "real" name as Roger, which has led many scholars to ascribe On Diverse Arts to the pen of Roger of Helmarshausen. Gearhart rehearses the arguments for this attribution and posits that the Vienna manuscript might have been an artist's working copy, but she does not by this mean to suggest that its potential readership was limited. As she importantly observes, "The knowledge of how things are made, of processes, materials, and labor expended, is a benefit to the patron as much as it is a requirement for the artist" (105). This is a crucial insight. Contemporary medieval art history tends to assign a central role to patrons, while also paying minutely close attention to the actual design and fabrication of images and objects. Few art-historical studies that engage in either or both of these practices try to account for how patrons might have gained the sophisticated understanding of technique and composition that such analyses presuppose or how they might have transmitted their desires and insights to craftsmen, who alone were the masters of the complicated techniques of manufacture. Gearhart's suggestion here that On Diverse Arts was read by both artists and patrons appears to be a way to cutting this Gordian knot; it certainly is a spur for further thought and research.
Theophilus and the Theory and Practice of Medieval Art is lavishly produced; its 30 color plates include not only multiple images of all the works Gearhart discusses in detail, but also of text pages from the various Theophilus manuscripts that structure her chapters. Such illustrations are a rarity in art-historical publications and are a testimony to how seriously Gearhart takes her manuscripts; one is of course used to art historians attending closely to the material nuances of illuminated manuscripts, but it is welcome indeed that Gearhart has extended this concern to text manuscripts. It is also welcome that Pennsylvania State University Press allowed her to do so. Given how common are complaints about presses in reviews of academic books, it is also worth noting that Gearhart's clearly written, entirely jargon-free book has been extremely carefully produced, with only the very occasional typographical error. I will not catalogue the few slips I found, but end simply by noting one that is fortuitous. In what I fondly hope is an intentional medievalizing conceit, the indented last paragraph on page 105 begins "he previous chapter examined skill as a pious moralized labor." The result of this slip (if slip it is) is to make it look as if space has been left for the rubricator to come through to supply the missing capital "T" to mark the beginning of the paragraph. How appropriate a reminder of the interrelationship of physical and intellectual processes of manufacture near the end of a book devoted to the interaction of reading, writing, and making in the twelfth century.