The specter of determinism and its implications for moral responsibility acted as a powerful motive on Kant's “critical” investigation of the structure of human reason and the limits of human knowledge. He was convinced that mathematical physics was on the right path and constituted an example that all natural sciences ought to follow. “I assert” – he wrote in 1786 – “that each special discipline concerning nature (besondere Naturlehre) can contain only so much genuine science as it contains mathematics.” But he stoutly opposed the facile opinion that modern physics can yield metaphysical conclusions concerning the subjects of greatest interest for mankind: God, freedom, and immortality. On such matters he “found it necessary to deny knowledge in order to make room for faith” (1787, p. xxx). Kant's faith was a distillation of Christianity. He understood it, however, not as a supernatural gift, but as the natural response of our “theoretical reason” to the living fact of “practical reason”, the cognitive echo of the voice of duty, so to speak.

Pious Christians had voiced qualms about modern natural philosophy since its inception. So Blaise Pascal, after making splendid con-tributions to geometry and physics, wrote c. 1660 about Cartesian mechanicism: “One ought to say in general: ‘;It happens by figure and motion’; for that is true. But to say which, and to compose the machine, is ridiculous, for it is useless and uncertain and wearisome.

‘Relativity theory’ or simply ‘Relativity’ is the standard name of two quite different, yet subtly related theories put forward by Albert Einstein in 1905 and 1915, respectively. The first, Special Relativity (SR), rescued the Maxwell equations from seemingly catastrophic experimental results by making deep changes in the basic concepts and laws of Newtonian mechanics. The second, General Relativity (GR), solved the problem of reconciling SR with Newton's theory of gravity by transcending them both. For those of us who cherish physicomathematical theories more for their inherent beauty than for their transient accuracy, SR and GR remain unmatched. Moreover, to this day, they have enjoyed tremendous empirical success. SR is corroborated daily in every high-energy lab. GR accounts for all the phenomena Newton classified as gravitational just as well or even better than his theory. Moreover, it provides an amazing gravitational explanation of other phenomena – such as the systematic shift in the spectrum of light from distant galaxies and the pervasive microwave background radiation –, which nobody even suspected c. 1910 and which would not easily fit in a Newtonian framework.

This is not the place to deal even superficially with the many fruitful applications of SR and GR. Our attention must go to their con-ceptual problems and their philosophical significance. The latter has been judged differently by different authors. In my view, it lies chiefly in the fact that both theories are exemplary cases of far-reaching conceptual change in fundamental physics, firmly rooted in the tradition they go beyond.

“Natural Philosophy consists in discovering the frame and operations of Nature, and reducing them, as far as may be, to general Rules or Laws, – establishing these rules by observations and experiments, and thence deducing the causes and effects of things …”. Sir Isaac Newton wrote this in the program he proposed to the Royal Society after he became its president in 1703. It is likely that by “the frame of nature” he and his readers meant its ultimate ingredients, the original components of bodies. One is tempted, however, to take the phrase as referring to the conceptual frame required for the mathematical description and explanation of natural phenomena. Newton himself put forward one such frame in the introductory sections of his masterpiece, the Principia of 1687 (Newton 1726, pp. 1–27). Without it one cannot make sense of the single, simple mathematical law by which he accounts in one breath for heavenly motions and free-fall. Newton's resolve to make explicit the structure underlying his physics sets him apart from the other founding fathers of modern physics; we must go back to Aristotle to find something of comparable breadth and depth. But Newton's conceptual frame, in stark contrast with Aristotle's, involves quantifiable, measurable attributes of things and was designed to fit the needs of mathematics and experiment. It remained the universal frame of physical inquiry until the advent of Einstein's Relativity, and, as we shall see in Chapter Three, it supplied a substantial part of the subject matter of Kant's critique of reason.

Like other volumes in “The Evolution of Modern Philosophy” series, this book is meant to introduce the reader to a field of contemporary philosophy – in this case, the philosophy of physics – by exploring its sources from the seventeenth century onward. However, while the modern philosophies of art, language, politics, religion, and so on seek to elucidate manifestations of human life that are much older and probably will last much longer than the philosophical will for lucidity, the modern philosophy of physics has to do with modern physics, an intellectual enterprise that began in the seventeenth century as a central piece of philosophy itself. The theory and practice of physics is firmly rooted in that origin, despite substantial changes in its informational contents, conceptual framework, and explicit aims. A vein of philosophical thinking about the phenomena of nature runs through the fourcentury- old tradition of physics and holds it together. This philosophy in physics carries more weight in the book than the reflections about physics conducted by philosophers. Our study of the evolution of the modern philosophy of physics will therefore pay much attention to the conceptual development of physics itself.

The book is divided into seven chapters. The purport and motivation of the first six are summarily described in the short introductions that precede them. The seventh and last chapter – “Perspectives and Reflections” – does not have an introduction, so I shall say something about it here.

Physics and Common Sense

Modern mathematical physics began in open defiance of common sense. Galileo declared – through his spokesman Salviati – that he could not “sufficiently admire the outstanding acumen” of the heliocentrist astronomers, who, “through sheer force of intellect,” had “done such violence to their own senses as to prefer what reason told them over that which sensible experience plainly showed them to the contrary” (EN VII, 355; Drake translation). Furthermore, he judged color and sound, heat and cold to be mere affections of the human senses, like the tickling one feels when a feather is introduced into one's nose, which, of course, lies not on the feather but on the nerves stimulated by it (1623, §48). The most conspicuous features by which we perceive and classify in everyday life the objects that surround us were thus pronounced mind-dependent or “subjective” and banished from the stock of notions that the new physics would employ to describe and understand the real, “objective” nature of things. Physical discourse retained many terms from ordinary language – arithmetic terms familiar in housekeeping and trade; geometric terms developed in carpentry, architecture, and land-surveying; chronometric terms used in lithurgy, seafaring, and, increasingly since the Renaissance, also in daily business; terms applicable to machines found in harbors and building sites –, which physicists sought only to define better and to apply with greater precision.

Physics and philosophy are still known by the Greek names of the Greek intellectual pursuits from which they stem. However, in the seventeenth century they went through deep changes that have conditioned their further development and interaction right to the present day. In this chapter I shall sketch a few of the ideas and methods that were introduced at that time by Galileo, Descartes, and some of their followers, emphasizing those aspects that I believe are most significant for current discussions in the philosophy of physics.

Three reminders are in order before taking up this task.

First, in the Greek tradition, physics was counted as a part of philosophy (together with logic and ethics, in one familiar division of it) or even as the whole of philosophy (in the actual practice of “the first to philosophize” in Western Asia Minor and Sicily). Philosophy was the grand Greek quest for understanding everything, while physics or “the understanding of nature (physis)” was, as Aristotle put it, “about bodies and magnitudes and their affections and changes, and also about the sources of such entities” (De Caelo, 268a1–4). For all their boasts of novelty, the seventeenth-century founders of modern physics did not dream of breaking this connection. While firmly believing that nature, in the stated sense, is not all that there is, their interest in it was motivated, just like Aristotle's, by the philosophical desire to understand.

From the perspective of eagle-eyed retrospection it is clear that when, during the early 1820s in Russia and then in western Europe a decade later, cholera first arrived in Europe, no one had any idea what had struck. Here was a disease that hit with astonishing ferocity, terrifying like only the plague and yellow fever before it, making its way from its origins in India by leaps and bounds along the main routes of commercial intercourse in an imprecise, yet identifiably northwesterly movement. And yet, it was a disease whose fundamental nature was long to remain concealed from even the most ardently attentive observers, a disease, as one German put it, “die wir wol nennen, aber nicht kennen.”

Ignorant of cholera's basic characteristics, medical expertise betrayed its helplessness in a luxuriant polymorphousness of preventive recommendations and cures, ranging from the harmless (steambaths, veils, fresh water, acupuncture, rubbings) to the gruesome: dousings with ice water, rectal injections of turpentine, extraction via pumps of inhaled miasma from the innards, cauterization of the stomach skin with boiling water and endless bleedings, which in the dehydration of the disease and its attendant coagulation, meant that blood had to be practically squeezed from the veins when it could be extruded at all. That homeopaths seized the opportunity to press their cause at least did little harm. Dr. Strack of Augsburg, convinced that different cloths and colors absorbed varying quantities of miasma, recommended white linen over black silk for the worried vain.

The first wave of cholera had broken unexpectedly over Europe, provoking at first reactions that were little more than the application of lessons learnt from past attacks of pestilential disease. Already during this first pandemic, however, it became clear that inherited quarantinist strategies would not necessarily prove effective this time. Examining their own experience and that of their predecessors, each nation underwent an epidemiological learning process that undercut the standing of quarantinism. In cholera's second phase, the half-century from the late 1830s up through Koch's discovery of the comma bacillus as the disease's cause and the gradual acceptance in official circles of its preventive implications during the late 1880s and early nineties, a similar process of experimentation, trial, error and the accumulation of experience continued. This increase in knowledge, though commonly shared among all nations, did not, however, lead in any automatic sense to uniform prophylactic strategies. States continued to take divergent approaches to cholera and other contagious diseases; indeed it may well have been that differences in national preventive tactics increased. Why, given a shared and increasingly accepted basis of knowledge, different tacks to a common problem persisted, is the question in need of an answer.

In the decades following the first epidemic, medical opinion remained largely unformed, while public health authorities continued the retreat from their initially strict quarantinism.

Venereal disease (for the purposes here primarily syphilis) complicated the problems of prophylaxis. Cholera and smallpox were acute epidemic diseases that quickly ran their course with evident external symptoms and a disabling effect on the victim. Identifying the stricken was therefore simple: they were usually in no position to object to precautions imposed and, indeed, had reason to seek the care that usually accompanied them. Syphilis, in contrast, whatever its origins, spread widely in the sixteenth century and by the nineteenth was endemic, afflicting a large (circa a tenth of adult urban males) fraction of the population. Its nature, with a surreptitious, protracted development and often painless symptoms, meant that it could be dissimulated and hidden by its sufferers, many of whom did not in fact realize they were ill, and the afflicted accordingly had ample opportunities to act as vectors of transmission. Symptoms neither necessarily drove the stricken to seek care nor prevented them from living life as usual. In fact, in the days before salvarsan and then penicillin, the protracted and unpleasant treatments that medicine had to offer were more inconvenient and debilitating than the disease. Because transmission generally required direct contact with permeable membranes, syphilis was in theory more easily preventable than diseases with more diffuse and multiple pathways of contagion. Because instinct nonetheless brought such membranes into frequent contact, in fact syphilis became among the most widely disseminated of transmissible ailments, spreading eventually the world over.

Smallpox sounded variations on the epidemiological themes first heralded with cholera. More endemic than the classic contagious diseases, smallpox was commonly regarded as among the worst of humanity's travails, an ailment that struck with blind disregard for sex or mode of life, favoring the young especially with its ravages, adding the humiliation of disfigurement for survivors of its other symptoms. That no one is spared either love or smallpox was the early modern version of our own, rather gloomier and mundane belief in the inevitability of the fiscus and the reaper. It was considered the most painful and debilitating of diseases, most lethal and costliest in its economic ravages. Even the plague would seem less destructive, was the grim calculation from early in the nineteenth century, were it not that we normally count children's lives only once they have survived smallpox.

While one of fate's hardest blows, however, smallpox was also the first contagious disease for which an effective, preventive medical intervention was developed and the first finally to be eradicated, a date set officially at 1979. Smallpox was thus an illness that allowed humanity to test its prophylactic prowess, the only shameful illness, as Lorain put it, because the one that could best be avoided. Inoculation, or variolation, and then vaccination equipped humans with preventive powers beyond the traditional techniques of breaking chains of transmission.

“One foot in the brothel, the other in the hospital,” goes the old saying, as applicable centuries ago as today. A universal for all mortals, disease is also an artifact of history. Patients racked by the fastigium of illness will take little comfort from the insight that they are suffering from a historical construct with only contingent objective reality, but scholars have found the multiplicity and mutability of illness irresistible. This diversity of signification attached to disease itself holds equally for the means employed to prevent and contain its spread. Why such precautions, the prophylactic strategies adopted in hopes of avoiding or ameliorating the ravages of epidemics, have varied dramatically among nations even though, in biological terms, the problem faced by each has been much the same is the question in search of an answer. Medical history is the immediate subject, but the ultimate purposes of this study extend beyond the precisely scientific. Since at least the era of absolutism, preventing and dealing with contagious and epidemic disease have together been one of the major tasks of states. When Cicero advised rulers to consider the salus populi as the highest law, he was thinking more of military security than sewers, but his dictum was soon to be interpreted as a reference to the public health.