Mechanical Philosophy

Gassendi and Descartes set the agenda for the next generation of natural philosophers, who accepted mechanical principles in general, believing that they had to choose between Gassendi's atomism and Descartes's plenism. Robert Boyle (1627–1691), Christiaan Huygens (1629–1695), and Isaac Newton (1642–1727), among the most prominent natural philosophers of the second half of the seventeenth century, developed their philosophies of nature in this context.

Boyle is best known for his attempt to incorporate chemistry within a mechanical framework. His corpuscular philosophy—which remained noncommittal on the question of whether matter is infinitely divisible or composed of indivisible atoms—was founded on a mechanical conception of matter. His reluctance to commit himself about the ultimate nature of matter reflected his concern about the atheism still associated with Epicureanism as well as his recognition that some questions lie beyond the ability of human reason to resolve. Material bodies are, according to Boyle, composed of extremely small particles, which combine to form clusters of various sizes and configurations. The configurations, motions, and collisions of these clusters produce secondary qualities, including the chemical properties of matter. Boyle conducted many observations and experiments aiming to demonstrate that various chemical properties can be explained mechanically. He performed an extensive series of experiments with the newly fabricated air pump to prove that the properties of air—most notably its "spring"—could be explained in mechanical terms.

Huygens followed Descartes in attempting to mathematize physics and the mechanical philosophy. He applied this approach to create a wave theory of light, a mathematical analysis of centrifugal force, and an improved theory of impact. He applied mathematics to physical problems far more successfully than had Descartes.

Newton, whose reputation rests on his achievements in mathematical physics and optics, accepted the mechanical philosophy from his student days at Cambridge University. A notebook written in the mid-1660s shows him thinking about natural phenomena in mechanical terms and designing thought experiments for choosing between Cartesian and Gassendist explanations of particular phenomena. A number of phenomena—including gravitation, the reflection and refraction of light, surface tension, capillary action, and certain chemical reactions—persistently resisted explanation in purely mechanical terms. Failing in the attempt to explain them by appeal to hypotheses about submicroscopic "aethers," Newton was led to the view that there exist attractive and repulsive forces between the particles composing bodies. This idea came to him from his alchemical studies. Newton's most notable discovery, the principle of universal gravitation, which provided a unified foundation for both terrestrial and celestial mechanics and which marks the culmination of developments started by Nicolaus Copernicus in the mid-sixteenth century, demanded a concept of attractive force. The concept of force, which seemed to some contemporaries to be a return to older theories of action at a distance that had been banished by the mechanical philosophy, enabled Newton to accomplish his stunning mathematization of physics and thereby fulfill the primary goal of the mechanical philosophy.

In the decades after Newton's death, the worst fears of the Christian mechanical philosophers of the seventeenth century came true. John Locke (1632–1704) argued for the reasonableness of Christianity, and his environmentalist analysis of the human mind—which grew directly from the ideas of the mechanical philosophers—implied the denial of the Christian doctrine of original sin. Deism and natural religion flourished both in England and on the Continent.

Debates about theories of generation became inextricably connected to philosophical and theological implications of the mechanical philosophy. While advocates of epigenesis tended to adopt vitalistic theories of life, theories that invoked some kind of nonmechanical entity to explain the properties of living things, another theory of generation, known as preformationism, seemed to be more compatible with the mechanical philosophy. Preformationism maintained that all living things existed within their ancestors, created by God at the beginning of life with a precise moment established for each one to emerge and become alive. Although preformationism was compatible with both divine providence and the doctrine of original sin, it also raised the specter of materialism that haunted the mechanical philosophy.

Some of the French philosophes, notably Julien Offroy de La Mettrie (1709–1751) and Paul-Henri-Dietrich d'Holbach (1723–1789), espoused atheistic materialism and also adopted vigorously anticlerical and antiecclesiastical views. David Hume (1711–1776) undermined the possibility of natural religion and a providential understanding of the world by demonstrating the invalidity of the standard arguments for the existence of God, particularly the argument from design that had played such a crucial role for the seventeenth-century mechanical philosophers. Newtonian mechanics rose to great heights, having shed the theological preoccupations of its creator. These developments culminated in the work of Pierre Simon Laplace (1749–1827), who articulated a clear statement of classical determinism and was able to demonstrate that the solar system is a gravitationally stable Newtonian system. When asked by Napoléon Bonaparte what role God played in his system, Laplace is reputed to have replied, "I have no need for that hypothesis."