A page from La Vie de Saint Denys, an illuminated manuscript presented by Gilles de Pontoise, abbot of Saint-Denis, to King Philippe V in 1317. ‘Below large-scale representations of the saint’s preaching, trials, tortures, and death,’ Eamon Duffy writes, are ‘vivid vignettes of life along the Seine’: ‘diminutive townspeople shop or borrow from moneylenders, physicians inspect flasks of their patients’ urine, animals are driven to slaughter, millers stagger under sacks of corn, workmen trundle wheelbarrows, and boatmen row goods and passengers up and down the river.’

Bibliothèque Nationale de France, Paris

A page from La Vie de Saint Denys, an illuminated manuscript presented by Gilles de Pontoise, abbot of Saint-Denis, to King Philippe V in 1317. ‘Below large-scale representations of the saint’s preaching, trials, tortures, and death,’ Eamon Duffy writes, are ‘vivid vignettes of life along the Seine’: ‘diminutive townspeople shop or borrow from moneylenders, physicians inspect flasks of their patients’ urine, animals are driven to slaughter, millers stagger under sacks of corn, workmen trundle wheelbarrows, and boatmen row goods and passengers up and down the river.’

Joel Kaye’s first book, Economy and Nature in the Fourteenth Century: Money, Market Exchange, and the Emergence of Scientific Thought (1998), was a revised version of his doctoral dissertation. Academic theses can make for dreary reading, but Kaye’s advanced a bold, sweeping, and closely argued theory, designed to explain a major shift in the thought of the later Middle Ages. In the dialectically structured thought-world of the late-thirteenth- and early-fourteenth-century universities (Latin scholae or schools, hence the adjective “scholastic”), the understanding of nature and the practice of natural science underwent profound change. A universe governed by hierarchically fixed and absolute values, Kaye argued, gave way to one characterized by change and motion. Observation and measurement displaced abstract speculation as the ways to understand nature, and that shift from deduction to quantification laid the foundations for a modern understanding of science.

That much was conventional enough. Kaye’s originality lay in his claim that the cause of this radical shift in thinking about nature was to be found in thought-patterns derived from the rapidly changing world of economics and transferred to scientific inquiry. Late medieval economic thinking was dominated by Aristotle’s discussion of value in the fifth book of his Nichomachean Ethics, where money features as the medium that makes possible comparison, measurement, and exchange between apparently unrelated things: How many shoes are equal to a house? What is the right proportion between goods and the labor that produces them?

Aristotle’s intellectual preeminence, newly restored to Europe from Arab sources, was of course foundational for the scholastic intellectual enterprise. But the rediscovery of Aristotle was not the only impetus to new thinking. In the booming mercantile economies of the thirteenth-century cities, and with the spread of the use of money, concepts rooted in the realities of the market were rapidly evolving.

Money itself was increasingly conceived as a dynamic medium connecting the constantly shifting values of commodities, labor, demand, and economic risk. The university thinkers responsible for the great shift in the understanding of the natural world, Kaye maintained, were themselves deeply immersed in this innovatory world of economics, as agents and administrators for their colleges and religious orders, and as members of the vibrant urban communities in which their universities were located. They were not only natural philosophers and theologians, but often also economic theorists and moralists, concerned with practical matters such as economic justice and especially the religious and ethical limits on usury in moneylending. And so the dynamic and shifting values of supply and demand in the marketplace, and the “monetization” of the cities, came to provide paradigms for a new and more dynamic understanding of the world as itself a dynamic realm best understood in terms of proportion, relativity, and mathematical measurement.

Kaye’s book was the product of deep immersion in the Latin writings of major and minor medieval thinkers from Albert the Great and Thomas Aquinas to Henry of Ghent, Peter John Olivi, and Nicole Oresme. It was widely praised for its fruitful crossing of disciplinary boundaries, bringing economic, political, and scientific thought together in mutually illuminating ways. But some reviewers thought that Kaye was working one single explanation too hard for plausibility, and suggested that the increasing use of money in the market was not sufficient to account for the profound intellectual changes he pointed to. Something more was needed “to explain why the mathematization of natural philosophy occurred when and where it did.”

After a decade and a half of further reading and rumination, Kaye’s new book seeks to offer that “something more.” A History of Balance revisits his earlier work, but refines, complicates, and broadens it. His new exploration of the shifts in medieval economic, political, medical, and scientific thought is presented as part of a wider inquiry, namely, “How do thoughts once unthinkable become thinkable? How do new images of the world and its workings take shape within intellectual cultures?” Kaye believes that there emerged after 1275 a novel and transformative conception of balance that revolutionized the thought of an age. A single if complex development, the emergence of “a new model of equilibrium” tranformed thinking across many disciplines—economics, political theory, natural sciences, and mathematics among others—bringing to them an entirely new level of sophistication, flexibility, and explanatory power.


The concept of “balance,” Kaye concedes, was rarely explicitly discussed in medieval scholastic writing, yet it underlay, he thinks, all that was most progressive and innovatory in the period. Balance had once been imagined as static, externally imposed, a “precondition of existence” built into nature, as Aristotle thought, or instilled into creation by God. But, Kaye maintains, balance came to be seen differently, as an intrinsic dimension of complex self-regulating systems, which had no need for “an exterior orderer or overarching ordering intelligence.”

In the “new equilibrium” that he thinks emerged in the late thirteenth century, relational thinking replaced hierarchical understandings of nature and society, fluidity replaced fixity, concern with motion and change replaced “the search for essences and perfections.” The underlying mathematical frame of thought moved from arithmetic to geometry, from balance achieved by mere addition or subtraction toward something more complex, less predictable. Estimation and approximation were now accepted not only as legitimate ways of knowing and measuring, but as the only ways human beings apprehend a physical and social world perpetually in flux.

Kaye’s first book focused on this new conceptual flexibility in economic thought. A History of Balance offers a fresh exploration of the transformation of discourse about moneylending—usury—in an extended examination of the writings of a late-thirteenth-century Franciscan thinker, Peter Olivi. The medieval church regarded moneylending at interest as a mortal sin, unnatural because money was intrinsically sterile and inert, and therefore could not “breed” profit. The balance between debt and repayment was both exact and static. If a man borrowed ten gold pieces he should return ten, neither more nor less. Olivi shared this inherited abhorrence of usury, but exploded the limitations of earlier theorists of the subject, because unlike them, he grasped that “all money is not equal.” Money loaned out by a merchant as venture “capital” (Olivi seems to have invented the word) was not the same as “simple” money.

Money designed for investment possessed “a kind of seminal cause of profit within itself”: in calculating the just level of repayment, multiple uncertainties had to be factored in—the passage of time, the loss of the use of money while it was on loan to others, shifts in demand for goods and services, the enhancement of value by transporting goods to markets, changes in the real value of coinage. Merchants “lived in the world of the probable” (Kaye’s italics), and this intrinsic uncertainty called for a new and less rigid way of calibrating just exchange, and ultimately a new concept of equilibrium. In due course, Kaye argues, that realization, rooted in the realities of the urban marketplace, radically revised scholastic theorizing on many fronts, as economists, moralists, and philosophers internalized the perception that economic value is always relative and approximative. Their thinking moved “from a world of points to a world of lines.”

But Kaye now insists that this new way of thinking was not confined to economics. A History of Balance pivots on the thought of the second-century physician, surgeon, and philosopher Galen of Pergamon, whose copious writings—perhaps as many as five hundred treatises—shaped and dominated Western medicine down to the eighteenth century, and in medieval universities formed the basis of the medical curriculum. Galen is Kaye’s principal intellectual hero, for in the ancient physician’s subtle handling of the notion of health as the balance in the human body of the four “humors”—wet and dry, hot and cold, or, more technically, sanguine and melancholic, choleric and phlegmatic—Kaye discerns an unparalleled sophistication and flexibility, determinative for the emergence of “the new equilibrium.” Medicine in thirteenth-century Europe, “simply trying to clear for itself a place at the learned table,” was considered inferior to disciplines like philosophy. Yet in the century that followed “it is the model of equilibrium proper to Galenic medicine, constructed around…approximations, constant flux, and fully relativized perspectives,” that would shape “the most advanced speculation in multiple fields of scholastic thought, including the field of natural philosophy” (i.e., science).

Galen’s contribution to the transformation of late medieval thought, Kaye argues, lay in the function of relativity in his deployment of the idea of balance. For Galen there could be no abstract or objectively “right” equilibrium of the humors in a living human body. In determining the “well-blended mixture” that constituted health for any particular individual, the physician must balance multiple considerations—individual temperament, physique, environment. In medicine, therefore, indeterminacy prevailed: all cases had to be handled “in a comparative and relative sense.” And it was the appropriation by other disciplines of this intuition—“Galenic relativity”—Kaye claims, that brought to birth “the new equilibrium.”

‘Galen collecting medicinal plants’; from Guillaume de Tignonville’s Les Dits Moraux des Philosophes, early fifteenth century

Russian National Library/Art Resource

‘Galen collecting medicinal plants’; from Guillaume de Tignonville’s Les Dits Moraux des Philosophes, early fifteenth century


In the course of the book, he explores the impact of this “new equilibrium” in many fields—the political thought of Marsilius of Padua, the mathematical and physical writings of Thomas Bradwardine and Nicole Oresme, the geological speculations of Jean Burridan—before tracing the collapse of the new equilibrium as more rigid social and political conditions prevailed. The more flexible ways of thinking, which had depended on the free air of self-regulating cities, withered.

The Galenic conception of balance in the literal physical body seemed especially adaptable to the body politic. Kaye believes that the early fourteenth century saw the emergence of new ways of thinking about the civic community, free of the metaphysical trappings of older and more rigid modes of thought. All medieval political and social discourse laid great stress on the notion of “the common good.” However, for an older generation of scholastic thinkers, like Albert the Great or Thomas Aquinas, the common good and hence the political and civic community, Kaye thinks, were conceived hierarchically and statically, by reference to God as the source of all good, since all government was traced by them to its derivation from the eternal law and government of God. Under the influence of Galenic relativity, however, progressive political thinkers increasingly conceived of the city or the commonwealth as a self-contained and self-regulating organism, like the Galenic human body, “capable of ordering itself in the absence of divine Intelligence and guidance.”

Kaye’s key exhibit here is Marsilius of Padua, whose anti-papal treatise Defensor Pacis (Defender of the Peace), published in 1324, mobilized concepts derived from Aristotle to present civil authority as derived from the collective voice of the citizens, and argued for elective monarchy (of which the Holy Roman Empire was of course a prime example) as the best form of government. The bulk of the Defensor consisted of a devastating deconstruction of the claim of the pope and the priesthood to any kind of secular authority, and resulted in its author’s excommunication. But it proved immensely influential, and Marsilius’s subordination of priestly power to the civic community has been seen as anticipating both the Protestant Reformation and the politics of secular modernity. For Kaye, however, its primary importance lies in its testimony to the pervasive influence of Galen, whose understanding of the animal body, he believes, “provided Marsilius with a way of understanding the possibilities of systematic self-equalization within the political body,” which would have resonated with the commercial and civic life of the cities in which he lived and worked.

This is a contentious claim, for although Marsilius was indeed a physician by training, Galen rates only a single passing mention in the Defensor. Kaye is untroubled by this silence, since he sees Galen’s influence working “for the most part beneath the level of conscious thought and expression.” Historians of political thought have hitherto failed to discern Galen’s rather than Aristotle’s influence on the biological metaphors of the Defensor, because they haven’t read enough Galen, just as they have failed to grasp the extent to which Galen’s view of nature is not only distinct from that of Aristotle, but “a notable advance over it.”

At this point in Kaye’s argument it is hard to avoid the suspicion that somewhere along the line, a working hypothesis has hardened into a doctrine. His analysis of Marsilius’s debt to Galen is based on the first thirteen of the Defensor’s fifty-two chapters. In them, he argues, Marsilius portrays the well-functioning political system as a reflection of the Galenic physical body, where tensions between competing private desires and functions create “a ‘tempered’ product,” the common good, “aided by the equalizing instrument of law and custom.”

The rest of Marsilius’s book is for Kaye an intellectual failure, in which Marsilius betrays his laudably rational, secular, and Galenic presentation of the civitas as an autonomous, self-contained, and self-regulating entity. Instead, Marsilius slides back into hierarchy, correlating the “principate,” the supreme elected authority, “with the unifying order of the Creator in the realm of all being.” In this godlike elevation of the princely function over the rest of the body politic, “the ‘co-aequalitas’ that is the essence of the Galenic body, where every part…continually interact[s] with each other in the production of systematic equality, is completely absent.”

But this seems to neglect the historic background of the Defensor, a polemic against the claims of the popes to the right to intervention in the life of the civitas and empire. Marsilius’s emphasis on the authority of the prince at the expense of the pope is a fundamental part of his book’s argumentative purpose. It is in any case anachronistic to see Marsilius’s thought as essentially secular. In a powerfully iconoclastic rereading of the Defensor, Professor George Garnett argued almost a decade ago that Marsilius should be understood neither as an Aristotelian political theorist nor as a proto-apologist for secular republicanism, but as an exponent of an apocalyptic and providentialist theology of Christian history. The proper setting for the Defensor, according to Garnett, lies in the fraught theological debates over the radical Franciscan doctrine of absolute apostolic poverty, which seemed to strike at the church’s wealth and power, and which Pope John XXII had condemned.

Marsilius, an ardent partisan for the Franciscans, saw the struggle of a corrupt papacy against the Holy Roman Emperors as the culminating phase of a millennial struggle between the forces of darkness and light. This radically religious rereading of the Defensor may or may not be correct, but it directs attention to the historical situation in which Marsilius wrote, and has at least as much claim to historical plausibility as Kaye’s contention that Marsilius’s book owes to Galen all that is of abiding value in it.

One of the most curious features of Kaye’s book is in any case its almost total lack of engagement with theology. Almost all the key figures in Kaye’s story were priests or monks, many of them professional theologians. Yet no one would suspect from A History of Balance that Peter Olivi’s principal claim to fame was as a religious activist, one of the earliest theorists of papal infallibility, and a leader of the Franciscan “Spirituals,” who preached and lived a doctrine of absolute poverty, who believed (like Marsilius) that he was living through a decisive phase in the great struggle against the Antichrist, and who devoted the last years of his life to the study of the Apocalypse. Kaye refers vaguely at one point to the condemnation by a commission of scholars from Paris of “certain of Olivi’s theological positions,” but we hear no more about them. Yet Olivi’s thinking about usury and the nature of money can hardly be understood in isolation from the consuming religious concerns that drove him to deal with the subject of money and value in the first place.

Since Kaye thinks that one of the principal effects of the new equilibrium was to liberate thinking from the need for “an exterior orderer or overarching ordering intelligence,” his lack of interest in theology is perhaps unsurprising. Yet if the “new equilibrium” was indeed as pervasive as Kaye argues, it must surely have had an impact on the dominant university discipline of theology. The dialectical structure of scholastic theology, its balancing of arguments pro and con, was embodied in one of its foundational texts, Peter Abelard’s Sic et Non (Yes and No), which set up contested questions for debate by juxtaposing apparently contradictory texts on each topic. That inescapably dialectical format cries out for exploration in terms of equilibrium and balance. If the same men were practicing theology while pioneering new lines of thought in other disciplines, can they have abandoned their commitment to “the new equilibrium” at the doors of the divinity schools?

On the face of it, such conceptual schizophrenia seems unlikely. Reviewing Kaye’s first book, in which many of the same thinkers and issues featured, William J. Courtenay suggested that many of the questions debated in the theology faculties of the thirteenth and fourteenth centuries would have been especially apt for treatment in terms of balance, such as the debates about merit and reward in the theology of grace, or the salvific value of Christ’s death as atonement for Adam’s sin. Thomas Bradwardine, who features in both of Kaye’s books as a scientist and mathematician, was also a major participant in precisely those theological debates about merit and reward, but A History of Balance has no interest in any such connections.

The problems of a monocausal explanation for the intellectual transformations of an entire age are highlighted in Kaye’s treatment of one of the most magnificent illuminated manuscripts produced in early-fourteenth-century France, La Vie de Saint Denys, presented by Gilles de Pontoise, abbot of Saint-Denis, to King Philippe V in 1317, now in the Bibliothèque Nationale in Paris. The text, by a monk of Saint-Denis, is an elaborate account of the life and martyrdom of Saint Denis, the legendary third-century missionary believed to have been Paris’s first bishop, and whose shrine was the abbey’s greatest treasure.

Illustrated by artists from a commercial Parisian workshop, the manuscript contains seventy-seven magnificent full-page illuminations, almost half of which include remarkable genre scenes set on the two main bridges of Paris (in reality wooden, but glamorized here into magnificent turreted stone structures). Vivid vignettes of life along the Seine occupy the lower register (about a third of each page) of thirty or so depictions of episodes from the saint’s life. Below large-scale representations of the saint’s preaching, trials, tortures, and death, diminutive townspeople shop or borrow from moneylenders, physicians inspect flasks of their patients’ urine, animals are driven to slaughter, millers stagger under sacks of corn, workmen trundle wheelbarrows, and boatmen row goods and passengers up and down the river.

These scenes of city life, “a vibrant picture of commercial Paris at the height of its prosperity,” rare in the art of the time, have excited much scholarly interest, and Kaye seizes on them as evidence of “the new equilibrium” in the urban community, reimagined as a “self-ordering system.” The manuscript’s remarkable juxtaposition of sacred and commercial scenes, Kaye thinks, presents starkly contrasting worldviews, “pointedly and inescapably different,” without even “a minimal degree of involvement or concern between ancient saint and the present-day city.”

On the one hand, the sacred scenes embody “a hieratic vision whose meaning depends upon the acceptance of a world of fixed values.” The city scenes below them, by contrast, portray an utterly different world, in which a “hierocratic, vertically integrated vision” has been replaced by a this-worldly “horizontal” perspective. “The sources of power and order in each are utterly distinct,” because beneath each lie different “models of equalization.” Saint Denis and the hierarchic world he represents have here nothing to offer the fourteenth-century city or its citizens. Kaye also attaches great weight to the absence from the city scenes of any authority figures, whether magistrate or monarch. In the Paris of these illustrations there is no need for any “overarching ordering intelligence, whether divine or royal. The city…orders itself.”

A moment’s reflection should make it clear that this is a deeply wrongheaded reading of the manuscript. Religion was integral to the life of Paris in the fourteenth century as of other great medieval towns, its civic and commercial life structured around the Christian year, its institutions staffed and serviced by parish clergy and the religious orders, its citizens eager participants in religious confraternities and guilds. Kaye treats the upper and lower registers of the illustrations not as the integrated scheme they manifestly are, but as if they emanated from utterly different patrons, makers, and thought-worlds. By his account, some backward-looking hierarch inspires the upper register of the page, some budding urban democrat the lower.

An illuminated book of this kind, the most lavish production of its time, cost a fortune, its pages made from the skins of flocks of sheep or calves, its colors beaten gold and powdered lapis lazuli. Yet Kaye invites us to believe that the head of France’s greatest monastery spent a king’s ransom on a gift for his monarch whose pictures undermined the very raison d’être of both church and crown, and signaled the autonomy of the city from them both. Medieval patronage just didn’t work like that, and medieval artists didn’t exercise that kind of subversive autonomy.

The book’s iconographic program must certainly have been dictated or approved by the powerful ecclesiastical corporation that paid for it, and a medieval commercial workshop had no motive to sabotage its clients’ interests. The Vie de Saint Denys manuscript was commissioned to impress upon the king the importance of Saint Denis and the abbey that housed his relics, not to suggest their irrelevance within a new (proto-secular) social order. It is overwhelmingly more likely that both those who made the images and those who first viewed them read the juxtaposition of saint and city in these pictures not as signaling their mutual incompatibility but, on the contrary, as pictorial shorthand for the claim that the people, the peace, and the prosperity of the king’s capital were sheltered under the protection of its patron saint.

But whatever its blind spots, A History of Balance is an audacious, learned, and fascinating book, likely to energize debate across many disciplines. An exploration of medieval thought that excludes theology can hardly claim comprehensiveness, and it may be doubted how far historians will be persuaded by Kaye’s “new equilibrium” as the universal key to the intellectual vitality of one of the most diverse and vibrant periods of medieval thought. But theology and art history aside, no one who works in the other fields this book engages with can afford to ignore it.