Newtonianism

Isaac Newton joined terrestrial and celestial mechanics together in 1687 with the publication of his Philosophiae naturalis principia mathematica, transforming the two kinds of physics into a single system oriented around the inverse-square law of gravitational attraction. Twenty years after this feat of mathematical synthesis, Newton contributed to framing experimental physics in his Opticks; or, A Treatise of the Reflections, Refractions, Inflections and Colours of Light, first published in 1704 and going through four editions by 1730. The Opticks laid out an experimentally derived geometry of light-rays, including the use of prisms to analyze white light into the colors of the spectrum and then resynthesize them into white light. The Opticks also included a final section of “queries” containing all the unfinished business of Newton's career. By means of this laundry list, Newton set the agenda for his eighteenth-century followers. Most importantly, he proposed that weightless ethers were the medium and material cause of forces and phenomena including light, heat, electricity, magnetism, gravitational attraction, and animal sensation.

Over the past three centuries, “Newtonianism” has meant several things. On the model of the Principia, it has meant a mathematical, synthetic approach to physics, and more specifically, the confirmation and promulgation of inverse-square laws of force. Important examples are John Mitchell's demonstration of an inverse-square law for magnetic force (1750) and Charles Augustin Coulomb's announcement of an inverse-square law governing electrical attraction and repulsion (1785–1789). Eighteenth- and nineteenth-century Newtonians also developed the field of astronomy by testing Newton's law of gravitational attraction against new observations and resolving apparent conflicts. Alexis-Claude Clairaut's explanation of the motion of the lunar apogee (1749) and his accurate prediction of the return of Halley's comet in 1759 confirmed and vindicated Newtonian astronomy. These efforts gave rise to an increasingly complex picture of the mutual gravitational influences of celestial bodies. The culmination of eighteenth and early nineteenth century Newtonian astronomy was Pierre-Simon Laplace's Traité de mécanique céleste (1798–1827), in which Laplace used Newton's law of gravitation to develop a complete theory of the solar system, taking into account complexities such as the perturbations in the orbits of the planets and the satellites caused by their mutual attraction.

On the model of the Opticks, meanwhile, Newtonianism has meant an inductive, experimental approach to physics. Users of this meaning of the word cite Newton's promise, in the preface to the Principia, to “feign no hypotheses.” An example of a Newtonian in this sense is Benjamin Franklin, who presented his electrical science as one founded in experimental tinkering rather than theory. His followers and historians have likened him, on that basis, to the Newton of the Opticks, the empirical essayer and querist. The empiricist meaning of Newtonianism has also referred, more specifically, to the use of analysis and synthesis experiments, for which Newton's investigations of white light served as the paradigm. The leading eighteenth-century example of experimental Newtonianism in this sense is Antoine-Laurent Lavoisier's analysis of water into hydrogen and oxygen and his resynthesis of these elements into water (1785).

The Opticks gave rise, finally, to a third meaning of Newtonianism, to describe the eighteenth- and nineteenth-century research program of so-called imponderables fluids that grew from Newton's hypothesis regarding force-bearing ethers. The hypothesis that an imponderable fluid medium carried each force informed theories of electricity, magnetism, heat, and light well into the nineteenth century. An eighteenth-century example of a fluid theory was Franklin's account of electricity, according to which a weightless electrical fluid, whose particles were mutually repulsive, permeated common matter, balancing the mutual attraction of its particles. Electrical effects accordingly resulted from the depletion (negative charge) or overabundance (positive charge) of the electrical fluid in a body. Another important eighteenth-century example was Joseph Black's understanding of heat. Black noticed that it took a great deal of heat simply to melt ice, without changing its temperature. He gave the name “latent heat” to the thermal fluid that seemed to disappear during phase changes, and he distinguished the quantity of this fluid in a heated object from its density, defining temperature as density of heat. An object's temperature depended, Black reasoned, upon its substance's capacity to contain the thermal fluid. These early notions of negative and positive electricity, of latent heat and of heat capacities, or specific heats, were thus informed by the Newtonian paradigm of imponderable fluids.

In addition to the work set forth in the Principia and the Opticks, another factor was crucial in shaping the meaning of Newtonianism, particularly during the eighteenth century: the contrast—partly genuine, but also overdrawn by Newton and his followers—between Newton's approach to physics and that of the French mathematician and natural philosopher René Descartes, to whose example Newton owed the beginnings of many of his ideas. Descartes notoriously allowed his rationalism and his commitment to rigorously mechanical explanations of natural phenomena to get the better of his physics. Based upon the principle that there could be no intelligible difference between matter and space, and on the conviction that physical events must have mechanical causes in the form of pushes between bits of matter, Descartes derived a picture of the universe as a great plenum in which all things were constrained to move in vortices. Newton's followers called Cartesian physics dogmatic, misguided, and arrogant in its claims to completeness. They pointed to Newton's abstention, in the Principia, from assigning a mechanical cause for gravitational attraction as the epitome of empiricist open-mindedness and humility. In celebration of what they took to be his epistemological modesty, Newtonians often referred to Newton's remark that he was merely collecting pretty pebbles beside the ocean of truth.

Leaving a gap at the heart of his system of mechanical causation, Newton allowed his disciples to fill in the metaphysics of their choosing. He himself wrote, in the queries to the Opticks, that natural phenomena arose not from mechanical causes, but from the will of a divine intelligence. This appeal to a final cause lying beyond the efficient ones pleased Enlightenment eulogists of Newton's mechanical system, who showed a remarkable tendency to cite its breaches. An example is David Hume's satisfaction that although Newton “seemed to draw off the veil from some of the mysteries of nature,” he also demonstrated “the imperfections of the mechanical philosophy,” restoring Nature's secrets “to that obscurity in which they ever did and ever will remain” (The History of England [1754–1762]). Voltaire, in his Lettres philosophiques (1734), popularized for a French audience the contrast between Newton's heroic acceptance, and Descartes's dogmatic refusal, of obscurity.

We now have four meanings of Newtonianism: a mathematical, synthetic approach to natural philosophy (particularly one founded in inverse-square laws of force); an inductive, experimental approach to natural philosophy (particularly one founded in analysis and synthesis experiments); the attribution of forces to weightless, force-bearing ethers or “imponderable fluids”; and the appeal to final causes, manifestations of the will of a divine intelligence, as the ultimate cause of natural phenomena, in contrast with Descartes's and his followers' strict adherence, in their natural philosophy, to mechanical causes.

The promulgation of Newtonianism coincided with an increasing interest in natural knowledge among the literate public. Some of the first people to teach courses of experimental physics were Newton's propagandists: Francis Hauksbee and John Theophilus Desaguliers, demonstrators at the Royal Society of London, and WillemJacob 'sGravesande, professor of mathematics at the University of Leiden, who was inspired by a meeting with Newton during a visit to London. These lecturers professed to translate Newton's physics from the language of mathematics into the language of experience, using demonstration experiments to make complicated ideas accessible to polite audiences. Popular written expositions of Newton's physics, including Desaguliers's and 'sGravesande's published lectures, emerged during the first third of the eighteenth century. Turning Newton's natural philosophy into a source of philosophical amusement, lecturers and authors established in the minds of their public a particular model of natural knowledge: quantitative and synthetic but also rigorously experimental; materialist and mechanist but also resting upon an underlying assumption that the ultimate causes in nature were final rather than efficient, reasons rather than mechanisms. The same model of knowledge took root in universities, academies, and technical and professional schools during the eighteenth century, beginning with the Royal Society, Cambridge University, and the University of Leiden, and spreading after about 1730 to France, Italy, Russia, and Sweden, where it mixed with continental traditions informed by the work of Descartes, Gottfried Leibniz, and others. Not only mathematicians and philosophers but doctors and engineers studied and taught Newtonian curricula by the end of the eighteenth century. Thus Newtonianism, with its several meanings, permeated the emerging professional and popular cultures of the Enlightenment.