In his "Mathematical Praeface" to the Elements of Euclid of 1570, Elizabethan polymath John Dee (1527–1608) expounded on the importance and utility of mathematics to all fields of human endeavor. Field after field, he argued, from those we would find obvious (like navigation) to those we would find arcane (astrology) or outlandish (thaumaturgike), would benefit from the systematic application of mathematics. Although Dee was promoting a role for mathematics that was just taking shape during his lifetime, his vision did indeed prove prophetic. Undoubtedly, one of the most striking features of intellectual life in the early modern period is the startling expansion in the scholarly and practical domains covered by mathematics.
Mathematics and Its Critics
Prior to the sixteenth century, mathematics in the West was a well-defined and circumscribed field consisting of two main branches: arithmetic, which had obvious practical applications in commerce and banking, and Euclidean geometry, which had few practical uses apart from astrology and, occasionally, optics. While mathematics was generally admired for the certainty and universality of its claims, the world as a whole, in keeping with Aristotelian tradition, was distinctly unmathematical, being governed by qualitative rather than quantitative rules. By the eighteenth century this view had been turned on its head: not only was an ever increasing number of fields being subjected to mathematical analysis, but the world itself had come to be understood as fundamentally mathematical in nature.
These developments were by no means a foregone conclusion in the sixteenth century; if anything, they seemed highly unlikely. For mathematics, far from being universally acknowledged as central to the intellectual and technological life of the age, was at the time being challenged as never before from various quarters.
Conservative critics, defending the established order of knowledge, challenged the truth claims of mathematics as incompatible with prevailing Aristotelian standards. Prominent among them were Italian philosopher Alessandro Piccolomini (1508–1578) and the Jesuit Benito Pereira (c. 1535–1610), who challenged the explanatory value of mathematical proofs. Proper scientific explanations, they argued with perfect Aristotelian orthodoxy, were causal arguments, proceeding from the true essence of objects to their properties. Mathematics, however, had no proper subject matter at all, and it could say nothing about the essential nature of physical objects. All mathematics could do was point to logical relations between hypothetical propositions, and thus it was a fundamentally inferior type of knowledge.
Mathematics did not fare much better among the new generation of reformers, who sought to uproot the Aristotelian framework and replace it with new conceptions of knowledge. In breaking the hold of Aristotelian standards on contemporary natural philosophy, many reformers found little use for mathematics. Its rigid procedures and unchanging truths seemed an unpromising basis for a radical reform of knowledge. The study of nature, many argued, should proceed through unmediated experience and systematic trial and error. The rigorous deductive reasoning characteristic of mathematics could only lead to predetermined and unvarying results. The maverick Italian philosopher Giordano Bruno (1548–1600), for example, argued that mathematics could only describe the external appearance of phenomena, but never penetrate their hidden secrets. Similarly, in England, Francis Bacon (1561–1626) in the Novum Organum insisted that mathematics "should only give limits to natural philosophy, not generate or beget it."
Mathematics did, of course, have many prominent defenders, ranging from the Jesuit Christopher Clavius (1537–1612) to Galileo Galilei (1564–1642) and René Descartes (1596–1650), each insisting in his way on the essential role of mathematics in any meaningful scheme of knowledge. But the very range of suggestions these and other natural philosophers offered for the role of mathematics in the general scheme of knowledge makes it clear that the fundamental questions raised by the challenges to mathematics did not go away. The critiques raised the fundamental questions that would guide the development of mathematics throughout the early modern period: what is mathematics, and how is it related to the natural world? The history of mathematics in this period is the story of the various answers that were given to these questions.
The World As Mirror of Mathematics
The fundamental answer to the critiques of mathematics was given by Galileo in his Assayer of 1623, when he wrote that the universe "is written in the language of mathematics." Galileo was expressing the widely held notion among practitioners that mathematics, far from being devoid of all subject matter as claimed by its critics, had the entire natural world as its object. But while most agreed that mathematics was closely integrated with the physical world, the precise nature of their relationship remained a matter of intense dispute.
One leading approach accepted the classical view of mathematics as a rigorous deductive science of number and magnitude. The universal laws of mathematics, in this view, were the fundamental laws that governed material reality. Thus when one is investigating mathematical and geometrical relationships, one is in fact investigating the basic structure of matter.
The chief promoter of this approach was René Descartes, who viewed mathematics as a fundamental rational law laid down by God for his creation. Once God, the divine architect, had set in motion his perfectly rational universe, it would henceforth operate forever in accordance with mathematical principles. Mathematical investigations are accordingly studies of the divine plan for the natural world, and the world is the direct expression of abstract mathematical principles.
Descartes's scientific work directly reflects this fundamental understanding. In his Meditations and the Discourse on Method, Descartes insisted that by following strict rational rules one could, in principle, follow God and "create" the world step by step. Rigorous rational deduction was therefore the key to knowledge of the natural world, and Descartes proceeded to demonstrate the effectiveness of this principle in short treatises on optics and the colors of the rainbow, which were attached to the early editions of the Discourse.
Descartes's most important contribution to mathematics was also a reflection of his religious and philosophical views. The Geometry was the founding text of analytic geometry and, like Descartes's other scientific treatises, was published as an appendix to the Discourse. In essence, the new field demonstrated the fundamentally mathematical nature of the physical world. Abstract algebraic relationships (that is, y=3Dax+b) were shown to have actual physical manifestations (in this case, a straight line). In pointing out these hidden relationships Descartes was unveiling the divine mathematical laws that governed the world. Mathematics, in this view, was a perfectly rational and logical web of relationships that determined the nature of physical reality.
Mathematics As the Mirror of the World
While Descartes was honing his analytical geometry, a very different mathematical approach, based on a very different understanding of the relationship of mathematics to the world, was being developed elsewhere in Europe. The use of infinitesimals, or "indivisibles" as they were most commonly called, in calculating lengths, areas, and volumes of geometrical figures was the most dramatic and important development in seventeenth-century mathematics. Fundamentally, the procedure involved reducing geometrical objects into an infinite number of their component parts: lines were viewed as an infinite collection of points, surfaces as made up of an infinite number of lines, and solids of surfaces. The length, area, or volume of the figure as a whole would then be calculated as the infinite sum of its elementary components.
The fundamental assumptions underlying this procedure were highly questionable and seemed to fly in the face of paradoxes that had been well known since antiquity. Descartes, who was much concerned with the perfect rational structure of mathematics, rejected infinitesimals and excluded them from the bounds of mathematics. Nevertheless, the effectiveness of this approach in reaching correct and previously unknown results was undeniable, and it was embraced enthusiastically by mathematicians across Europe. Thomas Hariot (1560–1621) and John Wallis (1616–1703) in England, Galileo and his disciples Bonaventura Cavalieri (c. 1598–1647) and Evangelista Torricelli (1608–1647) in Italy, Johannes Kepler (1571–1630) in Germany, and Blaise Pascal (1623–1662) in France were but a few of the most prominent practitioners of the new methods.
The infinitesimalist mathematicians' view of the relationship between mathematics and the world was, in many ways, the reverse of Descartes's approach. Whereas Descartes assumed that pure mathematical relationships governed the structure of matter, the infinitesimalists modeled mathematics on an intuition of the physical world. Geometrical bodies could be broken down into their indivisible components because, by analogy, physical bodies could be divided in the same way. As Cavalieri, whose Geometria Indivisibilibus was the most influential book about the theory and practice of indivisibles, wrote in his introduction, "plane figures should be conceived by us in the same manner as cloths are made up of parallel threads, and solids are in fact like books, composed of parallel pages."
The infinitesimalists' approach to mathematics drew much of its inspiration from the empiricist experimental philosophy that was gaining ground throughout Europe at this time. Much as the experimentalists sought to penetrate through external appearances and bring to light the inner structure of the material world, the new mathematicians sought to uncover the "inner structure" of geometrical figures, which in their view was the true cause of all geometrical relationships. Both groups, furthermore, adopted the imagery of geographical exploration as their guiding metaphor, presenting themselves as adventurous explorers on the hazardous seas of mathematics and natural philosophy.
Like their experimentalist colleagues, the infinitesimalists made the discovery of new and correct results the true test of their success, and like them they often adopted a methodology of trial and error in searching for the correct answers. This "experimental" approach to mathematics accounts for the infinitesimalists' relative disregard for the niceties of mathematical rigor and consistency. In their view, if a method produces true results, it must be fundamentally correct, and there was no point in spending too much time on clarifying the finer logical points. The most outspoken and unapologetic proponent of this approach was probably John Wallis, who advocated applying the experimentalists' "method of induction" to mathematics, in preference to traditional rigorous mathematical deduction.
While the new infinitesimalist approaches were in wide use in the seventeenth century, they were also seriously challenged in certain influential quarters. The issues at stake were not purely mathematical in nature, but involved wide-ranging philosophical, religious, and even political considerations. For one thing, the new approaches carried the taint of atomism—the ancient view that all material objects could be reduced to indivisible particles called "atoms" (from the Greek atomos, 'uncuttable'). Indeed there was no denying that the fundamental insights of the new mathematics and even its name strongly hinted that infinitesimalist mathematics was nothing but an expansion of atomism into mathematics.
This in turn led to a deeper difficulty: the suspicion that the new mathematics was based not just on atomism, but on materialism, which is the notion that the world was composed of nothing but matter, leaving no room for a providential spiritual realm. Geometry, after all, was often taken to be the very model of pure and abstract reasoning that governs the natural world. The notion that geometry itself, far from governing physical reality, is in fact a generalization of it, seemed to turn the proper hierarchy of mind and matter on its head, and challenge those who insisted that the world was ruled by a higher intelligence.
Finally, there was the question of the certainty of knowledge. Infinitesimalist mathematics seemed to be based on nothing more than a loose analogy with the physical world, trial and error, and a willful disregard for logical paradox. If even mathematics, that paragon of certain and unchanging knowledge, turned out to be so unsound, what hope could other, less rigorous fields have of attaining true knowledge?
In an age that still considered science, philosophy, and theology to be part of a single unified worldview, these criticisms cut deep. Descartes, concerned about the rational certainty of his method, excluded infinitesimal methods from proper mathematics. Even more significant was the reaction of the Society of Jesus, the most prominent religious order in Europe and the guardian of Catholic orthodoxy. Despite having among their members some of the most important and creative mathematicians in Europe, the Jesuits banned the teaching of infinitesimals from their educational institutions.
The Calculus and Beyond
The invention of the calculus by Isaac Newton (1642–1727) and Gottfried Wilhelm Leibniz (1646–1716) in the late seventeenth century was the most important development of early modern mathematics, and it quickly transformed the landscape of the field. The calculus took as its starting point the many practical techniquesand results achievedbythe infinitesimalist mathematicians, both in the determination of surfaces and volumes of geometrical figures, and in the calculation of tangents of curves. The fundamental insight of the calculus was that these two operations, calculating tangents (differentiation) and calculating surfaces and volumes (integration), are in fact the inverse of one another.
The importance of this discovery becomes clear when curves and geometrical figures are presented not as independent geometrical figures, but as expressions of algebraic formulations in the manner of analytic geometry. When presented in this manner, differentiation no longer deals with geometrical properties of particular geometrical objects, but becomes an abstract and general relationship between algebraic expressions. For example, one can say that the parabola expressed as y=3Dx2 describes the area under the line y=3D2x, and that y=3D2x expresses the tangent of the parabola y=3Dx2 at any point. But the relationship between the two algebraic expressions is no longer dependent on their particular geometrical representation: y=3D2x is simply the differential of y=3Dx2 and y=3Dx2is the integral of y=3D2x. The inverse relationship is a fundamental relationship between abstract algebraic expressions (or functions, as they came to be called later in the eighteenth century) independent of any particular geometric representation. Both Newton and Leibniz were quick to reduce the transformations back and forth between differentials and integrals (or "fluents" and "fluxions" as Newton called them) into systematic and reliable algorithms.
In the calculus, the two competing traditions of seventeenth-century mathematics were brought together. Although it clearly grew out of the techniques developed by infinitesimalist mathematicians, the calculus was equally dependent on the algebraic formulations of analytic geometry. Furthermore, the calculus detached the infinitesimalist methods from their dependence on an intuition of physical reality. If the older approaches could be viewed as growing out of an atomistic intuition of material reality, the calculus restored the primacy of abstract logical relationship to mathematics. Particular geometric figures could be seen as examples of these abstract algebraic relations, but these relations themselves were no longer dependent on any particular physical or geometrical instances.
Mathematics in the Enlightenment
The calculus, which positioned mathematics as both an abstract system of algebraic relationships and as intimately connected to the physical world, set the tone for eighteenth-century views of the field. The most eloquent formulation of attitudes toward mathematics in the Enlightenment was given by Jean Le Rond d'Alembert (1717–1783), in his "Preliminary Discourse" to the Encyclopédie, published in 1751. Whereas seventeenth-century practitioners viewed mathematics as either a generalization of material intuitions or as a universal law governing nature, for d'Alembert mathematics was necessarily both. On the one hand, he insisted, mathematics is clearly an abstraction from nature: it is nothing but the fundamental relationships among natural objects that are arrived at when the material features such as texture and color are stripped away. On the other hand, d'Alembert argued, the laws of nature are simply elaborations of mathematical relationships, arrived at by restoring matter's physical attributes to abstract disembodied mathematics. The world, then, according to d'Alembert, is fundamentally mathematical: mathematics is derived from the physical world, while the physical world is an extension of mathematical principles.
This view of an essentially mathematical universe manifested itself in the inclusion of an evergrowing number of scholarly fields that were brought under the sway of mathematics in this period. Years before, Galileo had already introduced mathematics into the study of falling bodies and statics, and he and his followers extended his work to the field of ballistics. Cartographic work was thoroughly mathematized in the seventeenth century, and Kepler and Newton transformed the ancient science of astronomy by extending the reach of mathematics from merely describing the motions of the heavens into the realms of celestial mechanics. In optics, Descartes's ingenious application of his "method" enabled him to explain such phenomena such as the formation of the rainbow with mathematical precision.
In the eighteenth century, a new generation of mathematicians, including the Bernoullis, Leonhard Euler (1707–1783), d'Alembert, Joseph-Louis Lagrange (1736–1813), and Pierre-Simon Laplace (1749–1827), among others, added increasingly precise theories of mechanics and argued famously about proper mathematical representations of abstract concepts such as vis viva, and concrete problems like the vibrations of strings and hydromechanics. Other fields that were seemingly less malleable for quantitative analysis, like doctrines of chance, or probability, and also the "moral" sciences, known today as social sciences, were also brought under the sway of mathematics, particularly in the work of the marquis de Condorcet (1743–1794). Institutionally, the eighteenth century saw mathematics gain a quickly growing foothold in newly established engineering and military colleges.
Epilogue
Unfortunately for d'Alembert and other promoters of the mathematical universe, rigorous mathematical analysis could not be easily derived from physical reality. Inconsistencies and paradoxes seemed to crop up repeatedly when mathematics was modeled on perceptions of the physical world, as critics of infinitesimal methods and the calculus, such as George Berkeley, were quick to point out. At the same time, the physical world proved to be far more varied and surprising than could ever be derived from bare mathematical principles.
Early in the nineteenth century the interdependence of mathematics and the physical world, so eloquently presented by d'Alembert, came to an end. In their work on the foundations of the calculus, mathematicians Bernhard Bolzano (1781–1848) and Augustin-Louis Cauchy (1789–1857) reformulated mathematical analysis as rigorous and logically self-consistent, a goal that had eluded their Enlightenment predecessors. They did so, however, at a price that would have seemed too heavy for d'Alembert and his colleagues: pure mathematics, in their scheme, was finally divorced from physical reality, existing in its self-enclosed Platonic realm.
The course and development of mathematics in the early modern period had come full circle. Criticized in the sixteenth century for being irrelevant to the developing sciences, mathematicians at the time had responded by forming a closer bond than ever before between their field and the physical world. Two and a half centuries later, in an attempt to save the identity and coherence of their field, mathematicians chose to sever those same conceptual ties, and establish mathematics in its own separate and insular domain.
Bibliography
Alexander, Amir. Geometrical Landscapes: The Voyages of Discovery and the Transformation of Mathematical Practice. Stanford, 2002.
Boyer, Carl B. The History of the Calculus and its Conceptual Development. New York, 1959.
Daston, Lorraine J. Classical Probability in the Enlightenment. Princeton, 1988.
Dear, Peter. Discipline and Experience: The Mathematical Way in the Scientific Revolution. Chicago, 1995.
Hankins, Thomas L. Science and the Enlightenment. Cambridge, U.K., and New York, 1985.
—AMIR ALEXANDER